When a buffer is folded in the split diff view and then removed from the
multibuffer (via `remove_excerpts_for_path`), the block map's
`folded_buffers` set retains a stale entry for that buffer ID. Later,
when the split view syncs folded state from the RHS display map to the
LHS, it tries to look up the companion mapping for this stale buffer and
fails because the companion no longer tracks a buffer that's been
removed from the multibuffer.
This originally caused a panic via `.expect()`. This PR:
1. Converts the panic into graceful handling by skipping unmapped
buffers.
2. Actively removes stale entries from `folded_buffers` so they don't
persist as ghost state — ensuring that if the same buffer is later
re-added, it won't incorrectly appear as folded.
3. Adds a regression test covering the full lifecycle: fold → remove →
split → re-add → assert both LHS and RHS report the buffer as not
folded.
Release Notes:
- Fixed a crash in split diff view when a folded buffer was removed and
re-added.
Antonio Scandurra
11bccbcdbf