branch: make the advice to force-deleting a conditional one

The error message we show when the user tries to delete a not fully
merged branch describes the error and gives a hint to the user:

	error: the branch 'foo' is not fully merged.
	If you are sure you want to delete it, run 'git branch -D foo'.

Let's move the hint part so that it is displayed using the advice
machinery:

	error: the branch 'foo' is not fully merged
	hint: If you are sure you want to delete it, run 'git branch -D foo'
	hint: Disable this message with "git config advice.forceDeleteBranch false"

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: rjusto

Documentation/config/advice.txt

@@ -45,6 +45,9 @@ advice.*::
 		Advice shown when linkgit:git-fetch[1] takes a long time
 		to calculate forced updates after ref updates, or to warn
 		that the check is disabled.
+	forceDeleteBranch::
+		Advice shown when a user tries to delete a not fully merged
+		branch without the force option set.
 	ignoredHook::
 		Advice shown if a hook is ignored because the hook is not
 		set as executable.

advice.c

@@ -47,6 +47,7 @@ static struct {
 	[ADVICE_DETACHED_HEAD]				= { "detachedHead", 1 },
 	[ADVICE_DIVERGING]				= { "diverging", 1 },
 	[ADVICE_FETCH_SHOW_FORCED_UPDATES]		= { "fetchShowForcedUpdates", 1 },
+	[ADVICE_FORCE_DELETE_BRANCH]			= { "forceDeleteBranch", 1 },
 	[ADVICE_GRAFT_FILE_DEPRECATED]			= { "graftFileDeprecated", 1 },
 	[ADVICE_IGNORED_HOOK]				= { "ignoredHook", 1 },
 	[ADVICE_IMPLICIT_IDENTITY]			= { "implicitIdentity", 1 },

advice.h

@@ -21,6 +21,7 @@ enum advice_type {
 	ADVICE_DETACHED_HEAD,
 	ADVICE_DIVERGING,
 	ADVICE_FETCH_SHOW_FORCED_UPDATES,
+	ADVICE_FORCE_DELETE_BRANCH,
 	ADVICE_GRAFT_FILE_DEPRECATED,
 	ADVICE_IGNORED_HOOK,
 	ADVICE_IMPLICIT_IDENTITY,

builtin/branch.c

@@ -24,6 +24,7 @@
 #include "ref-filter.h"
 #include "worktree.h"
 #include "help.h"
+#include "advice.h"
 #include "commit-reach.h"
 
 static const char * const builtin_branch_usage[] = {
@@ -190,9 +191,10 @@ static int check_branch_commit(const char *branchname, const char *refname,
 		return -1;
 	}
 	if (!force && !branch_merged(kinds, branchname, rev, head_rev)) {
-		error(_("the branch '%s' is not fully merged.\n"
-		      "If you are sure you want to delete it, "
-		      "run 'git branch -D %s'"), branchname, branchname);
+		error(_("the branch '%s' is not fully merged"), branchname);
+		advise_if_enabled(ADVICE_FORCE_DELETE_BRANCH,
+				  _("If you are sure you want to delete it, "
+				  "run 'git branch -D %s'"), branchname);
 		return -1;
 	}
 	return 0;