editor: Fix split diff spacer calculation for non-row-aligned patch groups

[timvermeulen]

Fixes a bug in the split diff spacer calculation when a patch group starts mid-row, sometimes causing extra spacers to be inserted.

spacer_blocks already explicitly handles the case where first_point isn't at the start of edit_for_first_point.old, but the while let Some(source_point) = source_points.next() loop that follows implicitly assumes that source_point is at the start of current_range, which in turn seems to be based on the assumption that current_range starts at the beginning of a row. As it turns out, current_range isn't guaranteed to start at the beginning of a row, which can sometimes lead to incorrect spacer blocks being inserted.

This addresses that by moving the existing if edit_for_first_point.old.start < first_point logic into the loop body as if current_edit.old.start < current_boundary in order to handle any non-row-aligned patch groups, not just the first one.

Here's an example of how this bug could manifest:

before.mov

After:

after.mov

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixed incorrect spacer blocks sometimes appearing in the split diff view when editing the file.

[cole-miller]

Thanks @timvermeulen!

[MrSubidubi]

@zed-industries/approved