GitLab

Currently _refresh handler caches data uses tip of tree
and caches data under the remote's master commit ID if
run without a 'commit' paramter.

This CL changes the handler to use the last cached 'master'
commit ID instead, so that refresh tasks for non-chromium-git
data can be run between refresh cycles.

BUG=None
R=kalman@chromium.org
NOTRY=True

Review URL: https://codereview.chromium.org/684113003

Cr-Commit-Position: refs/heads/master@{#302635}