Adding `--first-parent` ensures that only commits from the master branch
are used when showing the history. I've verified this change directly
by comparing the `rev-list` output for `703105c..97df5c45`:
```console
$ git rev-list --merges 703105c...97df5c4
97df5c4540b5976ada25
$ git rev-list --merges --first-parent 703105c...97df5c4
97df5c4540
```
Note: I've created this branch from 703105c so that I can merge `master`
into it and test `bash-it update` once merged.
babb1e6675