docs: Add use of Closes: tag for closing gitlab issues

This replaces to old Bugzilla: tag, which no longer makes sense because
we don't use bugzilla anymore.

Reviewed-by: Eric Anholt <eric@anholt.net> (v1)
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

docs/submittingpatches.html

@@ -75,11 +75,15 @@ if needed.  For example:
     platform.
 </pre>
 <li>A "Signed-off-by:" line is not required, but not discouraged either.
-<li>If a patch addresses a bugzilla issue, that should be noted in the
-patch comment.  For example:
+<li>If a patch addresses an issue in gitlab, use the Closes: tag
+For example:
 <pre>
-   Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89689
+    Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1
 </pre>
+<p>Prefer the full url to just <pre>Closes: #1</pre>, since the url makes it
+easier to get to the bug page from <pre>git log</pre></p>
+<b>Do not use the Fixes: tag for this!</b> Mesa already uses Fixes for something else.
+
 <li>If a patch addresses a issue introduced with earlier commit, that should be
 noted in the patch comment.  For example:
 <pre>