As chrome browser infra works to improve CQ times, we would like to
easily glance at builds and see how many new commits were checked out.
Currently you have to open up bot_update stdout to see the previous
HEAD revision and go from there. Having this conveniently outputted
would be very convenient.

If the bot has previously checked out the repo, output which revision
and commit position were checked out.

example LED build showing new output: https://ci.chromium.org/swarming/task/51592e46d6a70e10?server=chromium-swarm.appspot.com

Change-Id: Ib8a6f152a593446ce4082477bc9ada44f1fd1fc3
Bug: 1170568
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2644829Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>