docs: Tell people how to easily generate the Fixes lines.

v2: Include '-s' to suppress the diff.
v3: use the git config command (Ken), use < (Eric)

Reviewed-by: Matt Turner <mattst88@gmail.com> (v1)
Acked-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

docs/submittingpatches.html

@@ -85,6 +85,7 @@ noted in the patch comment.  For example:
 <pre>
    Fixes: d7b3707c612 "util/disk_cache: use stat() to check if entry is a directory"
 </pre>
+<li>You can produce those fixes lines by running <pre>git config --global pretty.fixes 'Fixes: %h ("%s")'</pre> once and then using <pre>git show -s --pretty=fixes &lt;sha1&gt;</pre>
 <li>If there have been several revisions to a patch during the review
 process, they should be noted such as in this example:
 <pre>