More work on create branch command

Added logic to check that a branch from a particular version
has not already been created. Also a bunch of small miscellaneous stuff.

TEST=run_tests.sh
BUG=chromium:980346

Change-Id: Ie948c2c5633eafe5a9ab619a2f1a56017dcd977d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/1690639
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Reviewed-by: Evan Hernandez <evanhernandez@chromium.org>
Commit-Queue: Jack Neus <jackneus@google.com>
Tested-by: Jack Neus <jackneus@google.com>
7 files changed