isort: format codebase

Change-Id: I6f11d123b68fd077f558d3c21349c55c5f251019
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/383715
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>

git_refs.py

@@ -13,8 +13,10 @@
 # limitations under the License.
 
 import os
-from repo_trace import Trace
+
 import platform_utils
+from repo_trace import Trace
+
 
 HEAD = "HEAD"
 R_CHANGES = "refs/changes/"