$ git checkout <name>

may be interpreted by git as checking out a file named <name>
if <name> has slashes. Thus,

$ git checkout infra/config

ends up checking out infra/config directory of the (typically)
master branch that was checked out before. To avoid confusion,
we have to add '--' separator between branch name and file names:

$ git checkout <name> --

This in turns allows to run CQ-based presubmit on infra/config branches.

R=nodir@chromium.org

Bug: 790738
Change-Id: I6ce31a8f0fbd66fd59ac7c2ea9cccd3ff97d1f0e
Reviewed-on: https://chromium-review.googlesource.com/802136
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Nodir Turakulov <nodir@chromium.org>