[release-tools] Only read from the chromium checkout in v8rel.

[release-tools] Only read from the chromium checkout in v8rel.

Don't create local branches or otherwise manipulate the
checkout. This reads refs from remote branches and
reads file contents using show. It is faster and requires
less bootstrapping and cleanup.

TBR=tandrii@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1060013003

Cr-Commit-Position: refs/heads/master@{#27640}