rename_ref: use lstat(2) when testing for symlink

The current check for symlinked reflogs was based on stat(2), which is
utterly embarrassing.

Fix it, and add a matching testcase.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

refs.c

@@ -792,7 +792,7 @@ int rename_ref(const char *oldref, const char *newref)
 	struct ref_lock *lock;
 	char msg[PATH_MAX*2 + 100];
 	struct stat loginfo;
-	int log = !stat(git_path("logs/%s", oldref), &loginfo);
+	int log = !lstat(git_path("logs/%s", oldref), &loginfo);
 
 	if (S_ISLNK(loginfo.st_mode))
 		return error("reflog for %s is a symlink", oldref);

t/t3200-branch.sh

@@ -104,4 +104,11 @@ test_expect_success \
         git-branch -m s/s s &&
         test -f .git/logs/refs/heads/s'
 
+test_expect_failure \
+    'git-branch -m u v should fail when the reflog for u is a symlink' \
+    'git-branch -l u &&
+     mv .git/logs/refs/heads/u real-u &&
+     ln -s real-u .git/logs/refs/heads/u &&
+     git-branch -m u v'
+
 test_done