This is the first step in reversing got_revision_mapping.

This CL:
- Adds a new configuration got_revision_reverse_mapping.
- Ensures only the old or the new configuration is used.
- Transforms the old to the new configuration within bot_update.

In follow up CLs we will migrate all clients to the new mapping, then delete the old and rename the new again.

Bug: 713356
Change-Id: Icdbd6a3748c6e5399b9c4867a3d8292973325160
Reviewed-on: https://chromium-review.googlesource.com/483479
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>