SubmittingPatches: use `--pretty=reference`

Since Git was taught the `--pretty=reference` option, it is no longer
necessary to manually specify the format string to get the commit
reference. Teach users to use the new option while keeping the old
invocation around in case they have an older version of Git.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -153,6 +153,12 @@ The "Copy commit summary" command of gitk can be used to obtain this
 format (with the subject enclosed in a pair of double-quotes), or this
 invocation of `git show`:
 
+....
+	git show -s --pretty=reference <commit>
+....
+
+or, on an older version of Git without support for --pretty=reference:
+
 ....
 	git show -s --date=short --pretty='format:%h (%s, %ad)' <commit>
 ....