Fix bugs in diff hunk highlighting

[maxbrunsfeld]

Fixes #18405

In #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:

    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

[zed-industries-bot]

	Messages
📖	This PR includes links to the following GitHub Issues: #18405 If this PR aims to close an issue, please include a Closes #ISSUE line at the top of the PR body.

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