patchtest: repo: refactor to use GitPython

patchtest: repo: refactor to use GitPython

The repo module currently uses a custom _exec() function in order to run
various git commands as part of the patchtest setup/test process. These
can more efficiently be done with the GitPython module, so use that
instead and reduce the amount of custom code to be maintained for
patchtest in the process. Some specifics replaced using GitPython:

- get branch list
- use repo.active_branch to determine current branch
- use execute() for checkout, merge check, abort, rev-parse, reset

The _exec() function is removed entirely with this change.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>