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

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