File marker '.dirty_bot_checkout' indicates that bot_update checkout has
started. Such marker is removed on successful checkout.

If such marker is present before creation, it means previous run wasn't
successful and may have left workspace in a bad state. In that case,
delete working directory and continue with a clean checkout.

R=apolito@google.com, ehmaldonado@chromium.org

Bug: 1074260
Change-Id: Ic089eedc26f7c269d5342fae366899571d8f8d41
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2186273
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>