Previously, if you used custom metadata keys in a test commit (like
GitRepo.AUTHOR_NAME), they would also be treated as filenames, and fail
because they aren't strings. (This wasn't a problem because it isn't
currently used in any tests.)

Bug: 808941
Change-Id: Id7c4fa5822741925beba591ea587bd8ebbf2e478
Reviewed-on: https://chromium-review.googlesource.com/901122Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>