bisect-kit: fix bug: cannot remove symlink when cleanup

BUG=None
TEST=manually

Change-Id: I871a5f9456f564b7e37c45f2ec3bb3748b758ae0
diff --git a/bisect_kit/git_util.py b/bisect_kit/git_util.py
index b783145..282e87b 100644
--- a/bisect_kit/git_util.py
+++ b/bisect_kit/git_util.py
@@ -490,7 +490,9 @@
   for untracked in list_untracked(git_repo, excludes=excludes):
     path = os.path.join(git_repo, untracked)
     logger.debug('delete untracked: %s', path)
-    if os.path.isdir(path):
+    if os.path.islink(path):
+      os.unlink(path)
+    elif os.path.isdir(path):
       shutil.rmtree(path)
     else:
       os.unlink(path)