cvetriager: optional pull command

Small change to allow the pull command on a given branch to be executed
only if DO_PULL is changed to True. Tests don't need to be modified
because do_pull is mocked.

BUG=chromium:1093363
TEST=python setup.py test

Change-Id: I610de13c597d65d6c4922924458e11fc5d2f54c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2300009
Commit-Queue: Wanda Mora <morawand@chromium.org>
Tested-by: Wanda Mora <morawand@chromium.org>
Reviewed-by: Zubin Mithra <zsm@chromium.org>
1 file changed