bisect-kit: fix pylint warnings

BUG=None
TEST=pylint

Change-Id: I5bdc7d4f491ef292c29d20226504d399245f6125
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bisect-kit/+/2497867
Tested-by: Kuang-che Wu <kcwu@chromium.org>
Commit-Queue: Zheng-Jie Chang <zjchang@chromium.org>
Reviewed-by: Zheng-Jie Chang <zjchang@chromium.org>
Auto-Submit: Kuang-che Wu <kcwu@chromium.org>
diff --git a/bisect_kit/git_util.py b/bisect_kit/git_util.py
index f621bde..9fdf7d9 100644
--- a/bisect_kit/git_util.py
+++ b/bisect_kit/git_util.py
@@ -93,6 +93,7 @@
 
   Args:
     git_repo: path of git repo.
+    initial_branch: the default branch after git init
   """
   if not os.path.exists(git_repo):
     os.makedirs(git_repo)