I just made a bunch of branches disappear from `git map-branches` by
creating a big cycle (none of them were a root, thus none was listed at
all). This CL makes `git map-branches` check for such cycles and report
them as warnings.

R=iannucci@chromium.org

Change-Id: I723b327282a1c90bc9da5438777653e48f8bfd1d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1528985
Auto-Submit: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>