This splits branch ref creation and landing. The ref now directly uses
heads (without pending) and for landing we use depot_tools.

For simplicity, the check for existing tags is removed, as it now
is very unlikely to happen again without gnumbd.

BUG=chromium:674448
NOTRY=true
TBR=tandrii@chromium.org

Review-Url: https://codereview.chromium.org/2584523002
Cr-Commit-Position: refs/heads/master@{#41716}