merge: allow "-" as a short-hand for "previous branch"

Just like "git checkout -" is a short-hand for "git checkout @{-1}" to
conveniently switch back to the previous branch, "git merge -" is a
short-hand for "git merge @{-1}" to conveniently merge the previous branch.

It will allow me to say:

    $ git checkout -b au/topic
    $ git am -s ./+au-topic.mbox
    $ git checkout pu
    $ git merge -

which is an extremely typical and repetitive operation during my git day.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1062,9 +1062,12 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (!allow_fast_forward && fast_forward_only)
 		die(_("You cannot combine --no-ff with --ff-only."));
 
-	if (!argc && !abort_current_merge && default_to_upstream)
-		argc = setup_with_upstream(&argv);
-
+	if (!abort_current_merge) {
+		if (!argc && default_to_upstream)
+			argc = setup_with_upstream(&argv);
+		else if (argc == 1 && !strcmp(argv[0], "-"))
+			argv[0] = "@{-1}";
+	}
 	if (!argc)
 		usage_with_options(builtin_merge_usage,
 			builtin_merge_options);