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>
diff --git a/git_refs.py b/git_refs.py
index 3187783..98ed1e2 100644
--- a/git_refs.py
+++ b/git_refs.py
@@ -15,7 +15,7 @@
 # limitations under the License.
 
 import os
-from trace import Trace
+from repo_trace import Trace
 import platform_utils
 
 HEAD      = 'HEAD'