Fix git_number windows unit tests

This fixes python2 unit tests (py3 works fine)

Change-Id: I9b968ba5d5f534166ce6c4fe59c8dfa4beb73021
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2121563
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
diff --git a/git_number.py b/git_number.py
index 91ba2c4..d8bca8c 100755
--- a/git_number.py
+++ b/git_number.py
@@ -142,7 +142,7 @@
 
   idx = os.path.join(git.run('rev-parse', '--git-dir'), 'number.idx')
   env = os.environ.copy()
-  env['GIT_INDEX_FILE'] = idx
+  env['GIT_INDEX_FILE'] = str(idx)
 
   progress_message = 'Finalizing: (%%(count)d/%d)' % len(DIRTY_TREES)
   with git.ProgressPrinter(progress_message) as inc: