Refactor CheckChangeOnCommit in PRESUBMIT
Refactor CheckChangeOnCommit in PRESUBMIT

Don't run git branch check on commit.  Being warned about not being
able to upload a CL is not necessary once you are already committing.

R=agable@chromium.org, mseaborn@chromium.org
BUG=522132
TEST=presubmit

Review URL: https://codereview.chromium.org/1302513005

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client