git: Rework side-by-side diff to use distinct buffers for the left-hand side (#44838)

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This PR reworks the (still feature-gated) side-by-side diff view to use
a different approach to representing the multibuffers on the left- and
right-hand sides.

Previously, these two multibuffers used identical sets of buffers and
excerpts, and were made to behave differently by adding a new knob to
the multibuffer controlling how diffs are displayed. Specifically, the
left-hand side multibuffer would filter out the added range of each hunk
from the excerpts using a new `FilteredInsertedHunk` diff transform, and
the right-hand side would simply not show the deleted sides of expanded
hunks. This approach has some problems:

- Line numbers, and actions that navigate by line number, behaved
incorrectly for the left-hand side.
- Syntax highlighting and other features that use the buffer syntax tree
also behaved incorrectly for the left-hand side.

In this PR, we've switched to using independent buffers to build the
left-hand side. These buffers are constructed using the base texts for
the corresponding diffs, and their lifecycle is managed by `BufferDiff`.
The red "deleted" regions on the left-hand side are represented by
`BufferContent` diff transforms, not `DeletedHunk` transforms. This
means each excerpt on the left represents a contiguous slice of a single
buffer, which fixes the above issues by construction.

The tradeoff with this new approach is that we now have to manually
synchronize excerpt ranges from the right side to the left, which we do
using `BufferDiffSnapshot::row_to_base_text_row`.

Release Notes:

- N/A

---------

Co-authored-by: cameron <cameron.studdstreet@gmail.com>
Co-authored-by: HactarCE <6060305+HactarCE@users.noreply.github.com>
Co-authored-by: Miguel Raz Guzmán Macedo <miguel@zed.dev>
Co-authored-by: Anthony <anthony@zed.dev>
Co-authored-by: Cameron <cameron@zed.dev>

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6 people authored and GitHub committed 2025-12-31 20:02:45 -05:00

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crates/buffer_diff/src/buffer_diff.rs

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