Remove unused GitCommandError import

Author: harshitayadavv

test/test_commit.py

@@ -14,7 +14,7 @@
 
 from gitdb import IStream
 
-from git import Actor, Commit, GitCommandError, Repo
+from git import Actor, Commit, Repo
 from git.objects.util import tzoffset, utc
 from git.repo.fun import touch