Closes #60424

## Problem

With the repro from #60424, staging the first deletion hunk marked the
second deletion as staged in the UI even though git still had it
unstaged, and clicking the (incorrectly shown) Unstage button inserted a
duplicate copy of the hunk's contents into the git index on every click.

The root cause is ambiguous hunk placement. The committed text contains
repeated `end\n\n` line runs, so the deletion hunks can "slide": more
than one placement produces a minimal diff. The uncommitted diff (HEAD
vs worktree) anchored the remaining deletion at one row while the
unstaged diff (index vs worktree), recomputed after the partial stage,
anchored the same logical deletion at a different row. Everything that
correlates hunks across those two diffs assumes they agree on positions:

- the secondary-status matching in `hunks_intersecting_range_impl` found
no unstaged hunk at the uncommitted hunk's rows and reported it as
staged (`NoSecondaryHunk`);
- the worktree→index projection in `compute_uncommitted_index_edits`
treated the hunk's position as unchanged text and, on unstage, inserted
the hunk's HEAD content at an index position that already contained it,
duplicating it on every request.

## Fix

Upgrade `imara-diff` from 0.1.8 to 0.2.0 and run
`Diff::postprocess_lines` (imara-diff's port of git's xdiff
slider/indent heuristic) after computing hunks in `buffer_diff`. This
canonicalizes the placement of ambiguous hunks based only on their local
content, so diffs of the same buffer against different base texts anchor
the same logical change at the same rows. A bonus is that Zed's hunk
placement now matches `git diff`'s output for such cases (git has used
the indent heuristic by default since 2.11).

As defense in depth, `compute_uncommitted_index_edits` now drops a
pure-insertion index edit whose content is already present at the target
position, so a stale secondary status can no longer duplicate index
content.

The imara-diff 0.2 API removed the `Sink` trait and the top-level
`diff()` function, so the other call sites (`language/text_diff.rs`,
`zeta_prompt/udiff.rs`, `edit_prediction_metrics/reversal.rs`) are
migrated mechanically to `Diff::compute` + `hunks()` with unchanged
behavior (0.2 also renamed `lines_with_terminator` to `lines` and
changed the default `&str` tokenization to include terminators; the
unified-diff builders keep terminator-less tokens via `str::lines()`).

## Testing

- New regression test `test_staging_hunks_with_ambiguous_placement`
replays the exact repro from #60424 (same file contents): stages the
first deletion, asserts the remaining hunks keep their unstaged status
and the index matches exactly, then issues repeated unstage requests for
the unstaged hunk and asserts the index is unchanged. Before the fix
this test showed the second hunk flipping to staged and the index
growing by one copy of the deleted block per unstage request.
- `buffer_diff`, `language`, `zeta_prompt`, `edit_prediction_metrics`,
`multi_buffer`, `git_ui`, `editor`, and the `project` integration suite
pass.
- One test expectation updated: `editor::test_fold_function_bodies`
asserted the old placement of an ambiguous deletion (blank line before
comment); the canonicalized placement (comment before blank line)
matches what `git diff` produces for the same texts.

Release Notes:

- Fixed staging a hunk sometimes marking a different hunk as staged (and
subsequent unstaging corrupting the git index) when the diff contained
repeated lines
([#60424](https://github.com/zed-industries/zed/issues/60424)).

---------

Co-authored-by: Cole Miller <cole@zed.dev>