Show error for 'git merge' with unset merge.defaultToUpstream

'git merge' can be called without any arguments if merge.defaultToUpstream
is set. However, when merge.defaultToUpstream is not set, the user will be
presented the usage information as if he entered a command with a wrong
syntaxis. Ironically, the usage information confirms that no arguments are
mandatory.

This adds a proper error message telling the user why the command failed. As
a side-effect this can help the user in discovering the possibility to merge
with the upstream branch by setting merge.defaultToUpstream.

Signed-off-by: Vincent van Ravesteijn <vfr@lyx.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -1094,9 +1094,12 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		die(_("You cannot combine --no-ff with --ff-only."));
 
 	if (!abort_current_merge) {
-		if (!argc && default_to_upstream)
-			argc = setup_with_upstream(&argv);
-		else if (argc == 1 && !strcmp(argv[0], "-"))
+		if (!argc) {
+			if (default_to_upstream)
+				argc = setup_with_upstream(&argv);
+			else
+				die(_("No commit specified and merge.defaultToUpstream not set."));
+		} else if (argc == 1 && !strcmp(argv[0], "-"))
 			argv[0] = "@{-1}";
 	}
 	if (!argc)