[Updater] Fix formats and add comments

Test: NA
Change-Id: If5d8d0ea5903e1cf4c2cb62dd3802fb3706458cb
diff --git a/git_utils.py b/git_utils.py
index abfcea7..580b3f3 100644
--- a/git_utils.py
+++ b/git_utils.py
@@ -111,4 +111,5 @@
 
 
 def is_commit(commit):
+    """Whether a string looks like a SHA1 hash."""
     return bool(COMMIT_RE.match(commit))