Rework diff rendering to allow putting the cursor into deleted text, soft-wrapping and scrolling deleted text correctly (#22994)

Closes #12553

* [x] Fix `diff_hunk_before`
* [x] Fix failure to show deleted text when expanding hunk w/ cursor on
second line of the hunk
* [x] Failure to expand diff hunk below the cursor.
* [x] Delete the whole file, and expand the diff. Backspace over the
deleted hunk, panic!
* [x] Go-to-line now counts the diff hunks, but it should not
* [x] backspace at the beginning of a deleted hunk deletes too much text
* [x] Indent guides are rendered incorrectly 
* [ ] Fix randomized multi buffer tests

Maybe:
* [ ] Buffer search should include deleted text (in vim mode it turns
out I use `/x` all the time to jump to the next x I can see).
* [ ] vim: should refuse to switch into insert mode if selection is
fully within a diff.
* [ ] vim `o` command when cursor is on last line of deleted hunk.
* [ ] vim `shift-o` on first line of deleted hunk moves cursor but
doesn't insert line
* [x] `enter` at end of diff hunk inserts a new line but doesn't move
cursor
* [x] (`shift-enter` at start of diff hunk does nothing)
* [ ] Inserting a line just before an expanded hunk collapses it

Release Notes:


- Improved diff rendering, allowing you to navigate with your cursor
inside of deleted text in diff hunks.

---------

Co-authored-by: Conrad <conrad@zed.dev>
Co-authored-by: Cole <cole@zed.dev>
Co-authored-by: Mikayla <mikayla@zed.dev>
Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>
Co-authored-by: Michael <michael@zed.dev>
Co-authored-by: Agus <agus@zed.dev>
Co-authored-by: João <joao@zed.dev>

crates/editor/src/display_map.rs

@@ -30,8 +30,8 @@ use crate::{
     hover_links::InlayHighlight, movement::TextLayoutDetails, EditorStyle, InlayId, RowExt,
 };
 pub use block_map::{
-    Block, BlockBufferRows, BlockChunks as DisplayChunks, BlockContext, BlockId, BlockMap,
-    BlockPlacement, BlockPoint, BlockProperties, BlockStyle, CustomBlockId, RenderBlock,
+    Block, BlockChunks as DisplayChunks, BlockContext, BlockId, BlockMap, BlockPlacement,
+    BlockPoint, BlockProperties, BlockRows, BlockStyle, CustomBlockId, RenderBlock,
     StickyHeaderExcerpt,
 };
 use block_map::{BlockRow, BlockSnapshot};
@@ -54,7 +54,7 @@ use language::{
 use lsp::DiagnosticSeverity;
 use multi_buffer::{
     Anchor, AnchorRangeExt, MultiBuffer, MultiBufferPoint, MultiBufferRow, MultiBufferSnapshot,
-    ToOffset, ToPoint,
+    RowInfo, ToOffset, ToPoint,
 };
 use serde::Deserialize;
 use std::{
@@ -68,7 +68,7 @@ use std::{
 };
 use sum_tree::{Bias, TreeMap};
 use tab_map::{TabMap, TabSnapshot};
-use text::LineIndent;
+use text::{BufferId, LineIndent};
 use ui::{px, SharedString, WindowContext};
 use unicode_segmentation::UnicodeSegmentation;
 use wrap_map::{WrapMap, WrapSnapshot};
@@ -367,10 +367,14 @@ impl DisplayMap {
         block_map.unfold_buffer(buffer_id, self.buffer.read(cx), cx)
     }
 
-    pub(crate) fn buffer_folded(&self, buffer_id: language::BufferId) -> bool {
+    pub(crate) fn is_buffer_folded(&self, buffer_id: language::BufferId) -> bool {
         self.block_map.folded_buffers.contains(&buffer_id)
     }
 
+    pub(crate) fn folded_buffers(&self) -> &HashSet<BufferId> {
+        &self.block_map.folded_buffers
+    }
+
     pub fn insert_creases(
         &mut self,
         creases: impl IntoIterator<Item = Crease<Anchor>>,
@@ -716,13 +720,8 @@ impl DisplaySnapshot {
         self.buffer_snapshot.len() == 0
     }
 
-    pub fn buffer_rows(
-        &self,
-        start_row: DisplayRow,
-    ) -> impl Iterator<Item = Option<MultiBufferRow>> + '_ {
-        self.block_snapshot
-            .buffer_rows(BlockRow(start_row.0))
-            .map(|row| row.map(MultiBufferRow))
+    pub fn row_infos(&self, start_row: DisplayRow) -> impl Iterator<Item = RowInfo> + '_ {
+        self.block_snapshot.row_infos(BlockRow(start_row.0))
     }
 
     pub fn widest_line_number(&self) -> u32 {