'C-1 C-x v L' asks for revision and shows its log entry with diff (bug#38044)

* doc/emacs/maintaining.texi (VC Change Log): Explain the numeric prefix arg
of 'C-x v L' (vc-print-root-log).

* lisp/vc/vc-git.el (vc-git-print-log): Add command line option "-p"
when vc-log-view-type is 'with-diff'.
(vc-git-log-view-mode): Use long style when vc-log-view-type is 'with-diff'.

* lisp/vc/vc.el (vc-print-log-internal): Add optional arg 'type'.
(vc-log-internal-common): Use 'region-history-mode' when type is
'with-diff' and backend supports 'region-history-mode'.
(vc-print-root-log): Add optional arg 'revision'.  In interactive spec
read a revision when current-prefix-arg is 1.  Use current-prefix-arg
"as is" when it is a number.  Show revision in long style with diff
when limit is 1 and revision is non-nil.

Author: link0ff

doc/emacs/maintaining.texi

@@ -991,7 +991,10 @@ file listed on the current line.
 @file{*vc-change-log*} buffer showing the history of the entire
 version-controlled directory tree (RCS, SCCS, CVS, and SRC do not
 support this feature).  With a prefix argument, the command prompts
-for the maximum number of revisions to display.
+for the maximum number of revisions to display.  A numeric prefix
+argument specifies the maximum number of revisions without prompting.
+When the numeric prefix argument is @kbd{M-1}, the command prompts
+for the revision ID, and displays its log entry with a diff of changes.
 
   The @kbd{C-x v L} history is shown in a compact form, usually
 showing only the first line of each log entry.  However, you can type

etc/NEWS

@@ -993,6 +993,9 @@ With a prefix argument asks for a command, so for example,
 'C-u M-x vc-log-search RET git log -1 f302475 RET' will display
 just one log entry found by its revision number.
 
++++
+*** 'C-1 C-x v L' asks for a revision and shows its log entry with diff.
+
 *** 'C-x v =' can now mimic Magit's diff format.
 Set the new user option 'diff-font-lock-prettify' to t for that, see
 below under "Diff mode".

lisp/vc/vc-git.el

@@ -708,7 +708,7 @@ or an empty string if none."
       '(menu-item "Snapshot Stash" vc-git-stash-snapshot
 		  :help "Snapshot stash"))
     (define-key map [cr]
-      '(menu-item "Create Samed Stash" vc-git-stash
+      '(menu-item "Create Named Stash" vc-git-stash
 		  :help "Create named stash"))
     (define-key map [de]
       '(menu-item "Delete Stash" vc-git-stash-delete-at-point
@@ -1134,8 +1134,7 @@ If LIMIT is a revision string, use it as an end-revision."
     ;; If the buffer exists from a previous invocation it might be
     ;; read-only.
     (let ((inhibit-read-only t))
-      (with-current-buffer
-          buffer
+      (with-current-buffer buffer
 	(apply 'vc-git-command buffer
 	       'async files
 	       (append
@@ -1161,6 +1160,8 @@ If LIMIT is a revision string, use it as an end-revision."
                                                             "HEAD"
                                                           limit)))
                     (list start-revision)))
+                (when (eq vc-log-view-type 'with-diff)
+                  (list "-p"))
 		'("--")))))))
 
 (defun vc-git-log-outgoing (buffer remote-location)
@@ -1226,7 +1227,7 @@ log entries."
   (set (make-local-variable 'log-view-file-re) regexp-unmatchable)
   (set (make-local-variable 'log-view-per-file-logs) nil)
   (set (make-local-variable 'log-view-message-re)
-       (if (not (memq vc-log-view-type '(long log-search)))
+       (if (not (memq vc-log-view-type '(long log-search with-diff)))
 	   (cadr vc-git-root-log-format)
 	 "^commit *\\([0-9a-z]+\\)"))
   ;; Allow expanding short log entries.
@@ -1235,7 +1236,7 @@ log entries."
     (set (make-local-variable 'log-view-expanded-log-entry-function)
 	 'vc-git-expanded-log-entry))
   (set (make-local-variable 'log-view-font-lock-keywords)
-       (if (not (memq vc-log-view-type '(long log-search)))
+       (if (not (memq vc-log-view-type '(long log-search with-diff)))
 	   (list (cons (nth 1 vc-git-root-log-format)
 		       (nth 2 vc-git-root-log-format)))
 	 (append

lisp/vc/vc.el

@@ -2361,7 +2361,7 @@ or if PL-RETURN is `limit-unsupported'."
                         'help-echo "Show the log again, including all entries")))
 
 (defun vc-print-log-internal (backend files working-revision
-                                      &optional is-start-revision limit)
+                                      &optional is-start-revision limit type)
   "For specified BACKEND and FILES, show the VC log.
 Leave point at WORKING-REVISION, if it is non-nil.
 If IS-START-REVISION is non-nil, start the log from WORKING-REVISION
@@ -2377,7 +2377,7 @@ earlier revisions.  Show up to LIMIT entries (non-nil means unlimited)."
          (shortlog (not (null (memq (if dir-present 'directory 'file)
                                     vc-log-short-style))))
 	(buffer-name "*vc-change-log*")
-         (type (if shortlog 'short 'long)))
+         (type (or type (if shortlog 'short 'long))))
       (vc-log-internal-common
        backend buffer-name files type
        (lambda (bk buf _type-arg files-arg)
@@ -2393,7 +2393,7 @@ earlier revisions.  Show up to LIMIT entries (non-nil means unlimited)."
            (vc-call-backend bk 'show-log-entry working-revision)))
        (lambda (_ignore-auto _noconfirm)
 	 (vc-print-log-internal backend files working-revision
-                              is-start-revision limit)))))
+                              is-start-revision limit type)))))
 
 (defvar vc-log-view-type nil
   "Set this to differentiate the different types of logs.")
@@ -2416,7 +2416,12 @@ earlier revisions.  Show up to LIMIT entries (non-nil means unlimited)."
       (let ((inhibit-read-only t))
 	;; log-view-mode used to be called with inhibit-read-only bound
 	;; to t, so let's keep doing it, just in case.
-	(vc-call-backend backend 'log-view-mode)
+	(vc-call-backend backend
+                         (if (and (eq type 'with-diff)
+                                  (vc-find-backend-function
+                                   backend 'region-history-mode))
+                             'region-history-mode
+                           'log-view-mode))
 	(set (make-local-variable 'log-view-vc-backend) backend)
 	(set (make-local-variable 'log-view-vc-fileset) files)
 	(set (make-local-variable 'revert-buffer-function)
@@ -2475,13 +2480,26 @@ WORKING-REVISION and LIMIT."
     (vc-print-log-internal backend files working-revision nil limit)))
 
 ;;;###autoload
-(defun vc-print-root-log (&optional limit)
+(defun vc-print-root-log (&optional limit revision)
   "List the change log for the current VC controlled tree in a window.
 If LIMIT is non-nil, it should be a number specifying the maximum
 number of revisions to show; the default is `vc-log-show-limit'.
-When called interactively with a prefix argument, prompt for LIMIT."
+When called interactively with a prefix argument, prompt for LIMIT.
+When the prefix argument is a number, use it as LIMIT.
+A special case is when the prefix argument is 1, in this case
+it asks for the revision and shows it with its diff."
   (interactive
    (cond
+    ((eq current-prefix-arg 1)
+     (let* ((default (thing-at-point 'word))
+	    (revision (read-string
+		       (if default
+			   (format "Revision to show (default %s): " default)
+			 "Revision to show: ")
+		       nil nil default)))
+       (list 1 revision)))
+    ((numberp current-prefix-arg)
+     (list current-prefix-arg))
     (current-prefix-arg
      (let ((lim (string-to-number
 		 (read-from-minibuffer
@@ -2492,17 +2510,20 @@ When called interactively with a prefix argument, prompt for LIMIT."
        (list lim)))
     (t
      (list (when (> vc-log-show-limit 0) vc-log-show-limit)))))
-  (let ((backend (vc-deduce-backend))
-	(default-directory default-directory)
-	rootdir)
+  (let* ((backend (vc-deduce-backend))
+	 (default-directory default-directory)
+	 (with-diff (and (eq limit 1) revision))
+	 (vc-log-short-style (unless with-diff vc-log-short-style))
+	 rootdir)
     (if backend
 	(setq rootdir (vc-call-backend backend 'root default-directory))
       (setq rootdir (read-directory-name "Directory for VC root-log: "))
       (setq backend (vc-responsible-backend rootdir))
       (unless backend
         (error "Directory is not version controlled")))
     (setq default-directory rootdir)
-    (vc-print-log-internal backend (list rootdir) nil nil limit)))
+    (vc-print-log-internal backend (list rootdir) revision revision limit
+                           (when with-diff 'with-diff))))
 
 ;;;###autoload
 (defun vc-print-branch-log (branch)