Also parse COMMIT argument into a commit

This fixes invocations like

    $ git when-merged v1.0.0 master

Signed-off-by: Michael Haggerty <[email protected]>

Author: mhagger

bin/git-when-merged

@@ -468,7 +468,7 @@ def main(args):
     commit = args.pop(0)
     # Convert commit into a SHA1:
     try:
-        commit = rev_parse(commit)
+        commit = rev_parse('%s^{commit}' % (commit,))
     except Failure as e:
         sys.exit(e.message)