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/selections_collection.rs

@@ -323,8 +323,7 @@ impl SelectionsCollection {
         self.all(cx).last().unwrap().clone()
     }
 
-    #[cfg(any(test, feature = "test-support"))]
-    pub fn ranges<D: TextDimension + Ord + Sub<D, Output = D> + std::fmt::Debug>(
+    pub fn ranges<D: TextDimension + Ord + Sub<D, Output = D>>(
         &self,
         cx: &mut AppContext,
     ) -> Vec<Range<D>> {
@@ -332,9 +331,9 @@ impl SelectionsCollection {
             .iter()
             .map(|s| {
                 if s.reversed {
-                    s.end.clone()..s.start.clone()
+                    s.end..s.start
                 } else {
-                    s.start.clone()..s.end.clone()
+                    s.start..s.end
                 }
             })
             .collect()
@@ -921,7 +920,7 @@ pub(crate) fn resolve_selections<'a, D, I>(
     map: &'a DisplaySnapshot,
 ) -> impl 'a + Iterator<Item = Selection<D>>
 where
-    D: TextDimension + Clone + Ord + Sub<D, Output = D>,
+    D: TextDimension + Ord + Sub<D, Output = D>,
     I: 'a + IntoIterator<Item = &'a Selection<Anchor>>,
 {
     let (to_convert, selections) = resolve_selections_display(selections, map).tee();