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/hover_popover.rs

@@ -265,12 +265,9 @@ fn show_hover(
 
             let local_diagnostic = snapshot
                 .buffer_snapshot
-                .diagnostics_in_range(anchor..anchor, false)
+                .diagnostics_in_range::<_, usize>(anchor..anchor)
                 // Find the entry with the most specific range
-                .min_by_key(|entry| {
-                    let range = entry.range.to_offset(&snapshot.buffer_snapshot);
-                    range.end - range.start
-                });
+                .min_by_key(|entry| entry.range.len());
 
             let diagnostic_popover = if let Some(local_diagnostic) = local_diagnostic {
                 let text = match local_diagnostic.diagnostic.source {
@@ -279,6 +276,15 @@ fn show_hover(
                     }
                     None => local_diagnostic.diagnostic.message.clone(),
                 };
+                let local_diagnostic = DiagnosticEntry {
+                    diagnostic: local_diagnostic.diagnostic,
+                    range: snapshot
+                        .buffer_snapshot
+                        .anchor_before(local_diagnostic.range.start)
+                        ..snapshot
+                            .buffer_snapshot
+                            .anchor_after(local_diagnostic.range.end),
+                };
 
                 let mut border_color: Option<Hsla> = None;
                 let mut background_color: Option<Hsla> = None;
@@ -770,7 +776,7 @@ impl InfoPopover {
 
 #[derive(Debug, Clone)]
 pub struct DiagnosticPopover {
-    local_diagnostic: DiagnosticEntry<Anchor>,
+    pub(crate) local_diagnostic: DiagnosticEntry<Anchor>,
     parsed_content: Option<View<Markdown>>,
     border_color: Option<Hsla>,
     background_color: Option<Hsla>,
@@ -823,10 +829,6 @@ impl DiagnosticPopover {
 
         diagnostic_div.into_any_element()
     }
-
-    pub fn group_id(&self) -> usize {
-        self.local_diagnostic.diagnostic.group_id
-    }
 }
 
 #[cfg(test)]