Keep the last N git push traces.
Name them after the time when they were collected, and add a
README file to each one to provide some context to developers.

Bug: 955206
Change-Id: Ib5fcf2f78fb65f6ddd80a93619c14e1ef70c5564
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1595108
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>