Add option to rbt patch for automatically creating a Git commit

The option -c, --commit automatically creates a commit in the current local
branch with the summary, description, bugs, testing, and author information
fetched from the review request.

Testing done:
Tested with a new review request using the commands 'rbt patch -c
<review_request_id>' and 'rbt patch --commit <review_request_id>'.

Unit tests passed.

Reviewed at https://reviews.reviewboard.org/r/4865/