check_filename(): handle ":^" path magic

We special-case "git log :/foo" to work when "foo" exists in
the working tree. But :^ (and its alias :!) do not get the
same treatment, requiring the user to supply a
disambiguating "--". Let's make them work without requiring
the user to type the "--".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -141,6 +141,10 @@ int check_filename(const char *prefix, const char *arg)
 		if (!*arg) /* ":/" is root dir, always exists */
 			return 1;
 		prefix = NULL;
+	} else if (skip_prefix(arg, ":!", &arg) ||
+		   skip_prefix(arg, ":^", &arg)) {
+		if (!*arg) /* excluding everything is silly, but allowed */
+			return 1;
 	}
 
 	if (prefix)