The `ep split-commit --split-point` argument now accepts split point
kinds:

* fim -- next edit is right under the cursor.
* same-file-near -- next edit is close to the cursor.
* same-file-far -- next edit is more than 30-lines away from cursor.
* cross-file -- next edit is in another file.

This lets us create datasets partitioned by kind more efficiently
compared to filtering them after the fact.

Release Notes:

- N/A