SubmittingPatches: use gitk's "Copy commit summary" format

Update the suggestion in 175d38c ("SubmittingPatches: document how
to reference previous commits", 2016-07-28) on the format to refer
to a commit to match what gitk has been giving since last year with
its "Copy commit summary" command; also mention this as one of the
ways to obtain a commit reference in this format.

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: bbolli

Documentation/SubmittingPatches

@@ -122,9 +122,14 @@ without external resources. Instead of giving a URL to a mailing list
 archive, summarize the relevant points of the discussion.
 
 If you want to reference a previous commit in the history of a stable
-branch use the format "abbreviated sha1 (subject, date)". So for example
-like this: "Commit f86a374 (pack-bitmap.c: fix a memleak, 2015-03-30)
-noticed [...]".
+branch, use the format "abbreviated sha1 (subject, date)",
+with the subject enclosed in a pair of double-quotes, like this:
+
+    Commit f86a374 ("pack-bitmap.c: fix a memleak", 2015-03-30)
+    noticed that ...
+
+The "Copy commit summary" command of gitk can be used to obtain this
+format.
 
 
 (3) Generate your patch using Git tools out of your commits.