Remembering the current branch is a relic from the past
where no work-dir checkout was used. Now this doesn't give
much benefit and screws up the script if it was left in
a bad state (e.g. after a master restart).

TBR=tandrii@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/1002383002

Cr-Commit-Position: refs/heads/master@{#27196}