This way we only check out a previous state of the requested branch.
For example, if the history looks like:

* (merge) commit 4
|\
| * commit 3
| |
| * commit 2
|/
* commit 1

If a commit before 3 is requested, previously, commit 2 would be
selected. With --first-parent, we only follow the left path, and commit
1 is selected. This avoids checking out the intermediate state on a
feature branch.