[land-bot] Bail early if there's nothing to land

Summary:
In the case of trying to land something that already landed, we shouldn't waste time
running smoke tests on an empty patch. arc land will still bail, so this isn't a bug fix,
however we don't want to accidentally wait for re-runs while other changes are in the
land queue. This also avoids smoke testing when no changes were made in the automated
commits pipeline.

Depends on D5801

Test Plan:

git checkout master
git checkout <this-branch> land-patch.sh
./land-patch.sh --dry-run   # Bails early

git commit -m "TEST"
./land-patch.sh --dry-run   # Does not bail early

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5808