Refactor BufferDiff to allow multiple diffs to share the same base text buffer

[cole-miller]

This PR changes how base texts are managed by the buffer_diff crate, to enable keeping two diff entities alive that share the same base text buffer entity. Previously, each diff owned its own base text buffer and edited it when calling BufferDiff::set_snapshot, so the only way to reuse the same base text between two diffs was to have two independent buffers for it, which is pretty inefficient.

After this PR, each diff still has a base text buffer, but set_snapshot doesn't edit it. Instead, that responsibility moves into the caller. For updating the base text buffer, this PR also introduces a new pair of APIs, Buffer::snapshot_with_edits and Buffer::fast_forward, which allow us to move the parsing of the new base text into the background and then install the new syntax tree synchronously on the foreground.

The git store uses the low-level APIs set_snapshot and fast_forward directly, and manages the head text and index text buffers itself (garbage-collecting them when they're no longer needed); this enables adding an open_staged_diff API which returns a diff between the managed index buffer and the managed head buffer (the latter is also used for the uncommitted diff's base text). Other downstreams don't need to reuse a base text buffer, and those have been migrated to use the high-level set_base_text API, which now calls set_snapshot and fast_forward internally, with a guard to prevent concurrent updates.

Another change worthy of note is that we now always diff the old base text with the new base text to create snapshot_with_edits.

There are also some incidental bug fixes:

• Fixed an issue where a dangling weak unstaged diff could stick around in the git store forever
• Restored the IndexMatchesHead optimization that had become inoperative in the remote case
• Fixed a crash in the multibuffer due to the handling of BufferDiffEvent::LanguageChanged, which could cause the multibuffer to have transforms that were inconsistent with the diff base text.

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

Closes ZED-81P

Release Notes:

• Fixed a rare crash that could occur while using the uncommitted diff.

[maxbrunsfeld]

🚀