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/git/project_diff.rs

@@ -146,7 +146,7 @@ impl ProjectDiffEditor {
         let editor = cx.new_view(|cx| {
             let mut diff_display_editor =
                 Editor::for_multibuffer(excerpts.clone(), Some(project.clone()), true, cx);
-            diff_display_editor.set_expand_all_diff_hunks();
+            diff_display_editor.set_expand_all_diff_hunks(cx);
             diff_display_editor
         });
 
@@ -310,9 +310,11 @@ impl ProjectDiffEditor {
                     .update(&mut cx, |project_diff_editor, cx| {
                         project_diff_editor.update_excerpts(id, new_changes, new_entry_order, cx);
                         project_diff_editor.editor.update(cx, |editor, cx| {
-                            for change_set in change_sets {
-                                editor.diff_map.add_change_set(change_set, cx)
-                            }
+                            editor.buffer.update(cx, |buffer, cx| {
+                                for change_set in change_sets {
+                                    buffer.add_change_set(change_set, cx)
+                                }
+                            });
                         });
                     })
                     .ok();
@@ -1105,6 +1107,8 @@ mod tests {
         path::{Path, PathBuf},
     };
 
+    use crate::test::editor_test_context::assert_state_with_diff;
+
     use super::*;
 
     // TODO finish
@@ -1183,19 +1187,13 @@ mod tests {
             let change_set = cx.new_model(|cx| {
                 BufferChangeSet::new_with_base_text(
                     old_text.clone(),
-                    file_a_editor
-                        .buffer()
-                        .read(cx)
-                        .as_singleton()
-                        .unwrap()
-                        .read(cx)
-                        .text_snapshot(),
+                    &file_a_editor.buffer().read(cx).as_singleton().unwrap(),
                     cx,
                 )
             });
-            file_a_editor
-                .diff_map
-                .add_change_set(change_set.clone(), cx);
+            file_a_editor.buffer.update(cx, |buffer, cx| {
+                buffer.add_change_set(change_set.clone(), cx)
+            });
             project.update(cx, |project, cx| {
                 project.buffer_store().update(cx, |buffer_store, cx| {
                     buffer_store.set_change_set(
@@ -1225,15 +1223,17 @@ mod tests {
         cx.executor()
             .advance_clock(UPDATE_DEBOUNCE + Duration::from_millis(100));
         cx.run_until_parked();
+        let editor = project_diff_editor.update(cx, |view, _| view.editor.clone());
 
-        project_diff_editor.update(cx, |project_diff_editor, cx| {
-            assert_eq!(
-                // TODO assert it better: extract added text (based on the background changes) and deleted text (based on the deleted blocks added)
-                project_diff_editor.editor.read(cx).text(cx),
-                format!("{change}{old_text}"),
-                "Should have a new change shown in the beginning, and the old text shown as deleted text afterwards"
-            );
-        });
+        assert_state_with_diff(
+            &editor,
+            cx,
+            indoc::indoc! {
+            "
+                - This is file_a
+                + an edit after git addThis is file_aˇ",
+            },
+        );
     }
 
     fn init_test(cx: &mut gpui::TestAppContext) {