symbolic-ref: allow refs/<whatever> in HEAD

Commit afe5d3d5 introduced a safety valve to symbolic-ref to
disallow installing an invalid HEAD. It was accompanied by
b229d18a, which changed validate_headref to require that
HEAD contain a pointer to refs/heads/ instead of just refs/.
Therefore, the safety valve also checked for refs/heads/.

As it turns out, topgit is using refs/top-bases/ in HEAD,
leading us to re-loosen (at least temporarily) the
validate_headref check made in b229d18a. This patch does the
corresponding loosening for the symbolic-ref safety valve,
so that the two are in agreement once more.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-symbolic-ref.c

@@ -45,8 +45,8 @@ int cmd_symbolic_ref(int argc, const char **argv, const char *prefix)
 		break;
 	case 2:
 		if (!strcmp(argv[0], "HEAD") &&
-		    prefixcmp(argv[1], "refs/heads/"))
-			die("Refusing to point HEAD outside of refs/heads/");
+		    prefixcmp(argv[1], "refs/"))
+			die("Refusing to point HEAD outside of refs/");
 		create_symref(argv[0], argv[1], msg);
 		break;
 	default:

t/t1401-symbolic-ref.sh

@@ -27,11 +27,6 @@ test_expect_success 'symbolic-ref refuses non-ref for HEAD' '
 '
 reset_to_sane
 
-test_expect_success 'symbolic-ref refuses non-branch for HEAD' '
-	test_must_fail git symbolic-ref HEAD refs/foo
-'
-reset_to_sane
-
 test_expect_success 'symbolic-ref refuses bare sha1' '
 	echo content >file && git add file && git commit -m one
 	test_must_fail git symbolic-ref HEAD `git rev-parse HEAD`