Teach bot_update to remove partially deleted git repos.
Teach bot_update to remove partially deleted git repos.

Also add logging to 'remove', helps to debug issues with 'build.dead' filling
up with crap.

R=iannucci@chromium.org, hinoka@chromium.org
BUG=647046

Review-Url: https://codereview.chromium.org/2342973002

primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools