Fix bugs in diff hunk highlighting (#18454)

Fixes https://github.com/zed-industries/zed/issues/18405

In https://github.com/zed-industries/zed/pull/18313, we introduced a
problem where git addition highlights might spuriously return when
undoing certain changes. It turned out, there were already some cases
where git hunk highlighting was incorrect when editing at the boundaries
of expanded diff hunks.

In this PR, I've introduced a test helper method for more rigorously
(and readably) testing the editor's git state. You can assert about the
entire state of an editor's diff decorations using a formatted diff:

```rust
    cx.assert_diff_hunks(
        r#"
        - use some::mod1;
          use some::mod2;
          const A: u32 = 42;
        - const B: u32 = 42;
          const C: u32 = 42;
          fn main() {
        -     println!("hello");
        +     //println!("hello");
              println!("world");
        +     //
        +     //
          }
          fn another() {
              println!("another");
        +     println!("another");
          }
        - fn another2() {
              println!("another2");
          }
        "#
        .unindent(),
    );
```

This will assert about the editor's actual row highlights, not just the
editor's internal hunk-tracking state.

I rewrote all of our editor diff tests to use these more high-level
assertions, and it caught the new bug, as well as some pre-existing bugs
in the highlighting of added content.

The problem was how we *remove* highlighted rows. Previously, it relied
on supplying exactly the same range as one that we had previously
highlighted. I've added a `remove_highlighted_rows(ranges)` APIs which
is much simpler - it clears out any row ranges that intersect the given
ranges (which is all that we need for the Git diff use case).

Release Notes:

- N/A

Cargo.lock

@@ -3729,6 +3729,7 @@ dependencies = [
  "multi_buffer",
  "ordered-float 2.10.1",
  "parking_lot",
+ "pretty_assertions",
  "project",
  "rand 0.8.5",
  "release_channel",