rename local trace module

There is a standard Python "trace" module, so having a local trace.py
prevents us being able to import that.  Rename the module to avoid.

Change-Id: I23e29ec95a2204bb168a641323d05e76968d9b57
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/234832
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

git_config.py

@@ -44,7 +44,7 @@ else:
 from signal import SIGTERM
 from error import GitError, UploadError
 import platform_utils
-from trace import Trace
+from repo_trace import Trace
 if is_python3():
   from http.client import HTTPException
 else: