Show v8 bleeding edge revision in trunk and Chromium commit message.

This allows to map from a V8 Chromium roll to the bleeding_edge revision used for the roll.

This CL also improves some test code:
 - Don't loose assertion exceptions in mocked git on retry.
 - Remove an unused method

BUG=337167
LOG=n
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/139993006

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18778 ce2b1a6d-e550-0410-aec6-3dcde31c8c00