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/remote_server/src/remote_editing_tests.rs

@@ -86,9 +86,9 @@ async fn test_basic_remote_editing(cx: &mut TestAppContext, server_cx: &mut Test
         .await
         .unwrap();
 
-    change_set.update(cx, |change_set, cx| {
+    change_set.update(cx, |change_set, _| {
         assert_eq!(
-            change_set.base_text_string(cx).unwrap(),
+            change_set.base_text_string().unwrap(),
             "fn one() -> usize { 0 }"
         );
     });
@@ -150,9 +150,9 @@ async fn test_basic_remote_editing(cx: &mut TestAppContext, server_cx: &mut Test
         &[(Path::new("src/lib2.rs"), "fn one() -> usize { 100 }".into())],
     );
     cx.executor().run_until_parked();
-    change_set.update(cx, |change_set, cx| {
+    change_set.update(cx, |change_set, _| {
         assert_eq!(
-            change_set.base_text_string(cx).unwrap(),
+            change_set.base_text_string().unwrap(),
             "fn one() -> usize { 100 }"
         );
     });