refs: on symref reflog expire, lock symref not referrent

When locking a symbolic ref to expire a reflog, lock the symbolic
ref (using REF_NODEREF) instead of its referent.

Add a test for this.

Signed-off-by: David Turner <dturner@twopensource.com>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -3314,7 +3314,8 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
 	 * reference itself, plus we might need to update the
 	 * reference if --updateref was specified:
 	 */
-	lock = lock_ref_sha1_basic(refname, sha1, NULL, NULL, 0, &type, &err);
+	lock = lock_ref_sha1_basic(refname, sha1, NULL, NULL, REF_NODEREF,
+				   &type, &err);
 	if (!lock) {
 		error("cannot lock ref '%s': %s", refname, err.buf);
 		strbuf_release(&err);

t/t1410-reflog.sh

@@ -338,4 +338,14 @@ test_expect_failure 'reflog with non-commit entries displays all entries' '
 	test_line_count = 3 actual
 '
 
+test_expect_success 'reflog expire operates on symref not referrent' '
+	git branch -l the_symref &&
+	git branch -l referrent &&
+	git update-ref referrent HEAD &&
+	git symbolic-ref refs/heads/the_symref refs/heads/referrent &&
+	test_when_finished "rm -f .git/refs/heads/referrent.lock" &&
+	touch .git/refs/heads/referrent.lock &&
+	git reflog expire --expire=all the_symref
+'
+
 test_done