Add integration tests for existing local and remote branch when rebasing
Add fail-fast for new commits between rebased branch and new base

This commit should close #25 by comparing the SHA1 of the newly rebased
branch with the SHA1 of the new base. If they are the same, e.g. if the
PR has already been rebased, the Rebase command throws a Stop with an
informative error message.
Add integration test for no new commits fail-fast checkpoint