When syncing remote branches, checkout origin/<branch> instead of
<branch>, since there might be a local branch with that name.

Bug: 859703
Change-Id: I9a49b940f53f75a97e5458fdcf578c3b52b26de2
Reviewed-on: https://chromium-review.googlesource.com/1128271Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>