cvetriager: fix on create_new_cherry_pick_branch

Fixed create_new_cherry_pick_branch by allowing for the branch to be
checked-out to if it exists instead of raising an error for that reason.
Also added missing full-stops to comments.

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

Change-Id: I9684979e3f13d562df4e03e6264339902050464d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2300739
Commit-Queue: Wanda Mora <morawand@chromium.org>
Tested-by: Wanda Mora <morawand@chromium.org>
Reviewed-by: Zubin Mithra <zsm@chromium.org>
2 files changed