start: add a --HEAD alias

This makes it easy to use --H as a shortcut, and kind of matches the
use of storing HEAD as the revision.

Change-Id: I590bf488518f313e7a593853140316df98262d7e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/301163
Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
diff --git a/subcmds/start.py b/subcmds/start.py
index 25b229f..04589fb 100644
--- a/subcmds/start.py
+++ b/subcmds/start.py
@@ -44,7 +44,8 @@
                  help='begin branch in all projects')
     p.add_option('-r', '--rev', '--revision', dest='revision',
                  help='point branch at this revision instead of upstream')
-    p.add_option('--head', dest='revision', action='store_const', const='HEAD',
+    p.add_option('--head', '--HEAD',
+                 dest='revision', action='store_const', const='HEAD',
                  help='abbreviation for --rev HEAD')
 
   def ValidateOptions(self, opt, args):