depot_tools: Run git_common_test in Python 3

Bug: 1009809
Change-Id: Idfcbd26de3420798f092c7fa55a6126d7c389a8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1834317
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
diff --git a/git_rebase_update.py b/git_rebase_update.py
index fa5b5c4..e1d6b22 100755
--- a/git_rebase_update.py
+++ b/git_rebase_update.py
@@ -320,8 +320,8 @@
     else:
       root_branch = git.root()
       if return_branch != 'HEAD':
-        print("%r was merged with its parent, checking out %r instead." %
-              (return_branch, root_branch))
+        print("%s was merged with its parent, checking out %s instead." %
+              (git.unicode_repr(return_branch), git.unicode_repr(root_branch)))
       git.run('checkout', root_branch)
 
     # return_workdir may also not be there any more.