Optimistically update hunk states when staging and unstaging hunks (#25687)

This PR adds an optimistic update when staging or unstaging diff hunks.
In the process, I've also refactored the logic for staging and unstaging
hunks, to consolidate more of it in the `buffer_diff` crate.

I've also changed the way that we treat untracked files. Previously, we
maintained an empty diff for them, so as not to show unwanted
entire-file diff hunks in a regular editor. But then in the project diff
view, we had to account for this, and replace these empty diffs with
entire-file diffs. This form of state management made it more difficult
to store the pending hunks, so now we always use the same
`BufferDiff`/`BufferDiffSnapshot` for untracked files (with a single
hunk spanning the entire buffer), but we just have a special case in
regular buffers, that avoids showing that entire-file hunk.

* [x] Avoid creating a long queue of `set_index` operations when
staging/unstaging rapidly
* [x] Keep pending hunks when diff is recalculated without base text
changes
* [x] Be optimistic even when staging the single hunk in added/deleted
files
* Testing

Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <m@cole-miller.net>

Cargo.lock

@@ -2076,6 +2076,7 @@ name = "buffer_diff"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "clock",
  "ctor",
  "env_logger 0.11.6",
  "futures 0.3.31",