fix: avoid index-out-of-bounds panic in staging apply_selection

[SAY-5]

This Pull Request fixes/closes #2953.

It changes the following:

• NewFromOldContent::add_old_line and catchup_to_hunkstart no longer index old_lines past its length, so unstaging or discarding lines near the end of a diff returns the working content unchanged instead of aborting with index out of bounds.
• Added two unit tests against NewFromOldContent that exercise the previously panicking path.

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog