Merge branch 'hv/doc-commit-reference-style' into maint

A small doc update.

* hv/doc-commit-reference-style:
  SubmittingPatches: use gitk's "Copy commit summary" format
  SubmittingPatches: document how to reference previous commits

Author: gitster

Documentation/SubmittingPatches

@@ -121,6 +121,16 @@ its behaviour.  Try to make sure your explanation can be understood
 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)",
+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.