Fix panic when a file in a path-based multibuffer excerpt is renamed (#28364)

Closes #ISSUE

Release Notes:

- Fixed a panic that could occur when paths changed in the project diff.

Co-authored-by: Conrad Irwin <conrad.irwin@gmail.com>

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -1798,6 +1798,88 @@ fn test_set_excerpts_for_buffer(cx: &mut TestAppContext) {
     });
 }
 
+#[gpui::test]
+fn test_set_excerpts_for_buffer_rename(cx: &mut TestAppContext) {
+    let buf1 = cx.new(|cx| {
+        Buffer::local(
+            indoc! {
+            "zero
+            one
+            two
+            three
+            four
+            five
+            six
+            seven
+            ",
+            },
+            cx,
+        )
+    });
+    let path: PathKey = PathKey::namespaced(0, Path::new("/").into());
+    let buf2 = cx.new(|cx| {
+        Buffer::local(
+            indoc! {
+            "000
+            111
+            222
+            333
+            "
+            },
+            cx,
+        )
+    });
+
+    let multibuffer = cx.new(|_| MultiBuffer::new(Capability::ReadWrite));
+    multibuffer.update(cx, |multibuffer, cx| {
+        multibuffer.set_excerpts_for_path(
+            path.clone(),
+            buf1.clone(),
+            vec![Point::row_range(1..1), Point::row_range(4..5)],
+            1,
+            cx,
+        );
+    });
+
+    assert_excerpts_match(
+        &multibuffer,
+        cx,
+        indoc! {
+        "-----
+        zero
+        one
+        two
+        -----
+        three
+        four
+        five
+        six
+        "
+        },
+    );
+
+    multibuffer.update(cx, |multibuffer, cx| {
+        multibuffer.set_excerpts_for_path(
+            path.clone(),
+            buf2.clone(),
+            vec![Point::row_range(0..1)],
+            2,
+            cx,
+        );
+    });
+
+    assert_excerpts_match(
+        &multibuffer,
+        cx,
+        indoc! {"-----
+                000
+                111
+                222
+                333
+                "},
+    );
+}
+
 #[gpui::test]
 fn test_diff_hunks_with_multiple_excerpts(cx: &mut TestAppContext) {
     let base_text_1 = indoc!(