buffer_diff: Remove git2 dependency from buffer_diff (#56013)

Split out of https://github.com/zed-industries/zed/pull/53453

Release Notes:

- N/A or Added/Fixed/Improved ...

Co-authored-by: Kieran Freitag <kfreitag@kieran.ca>

crates/editor/src/editor_tests.rs

@@ -11790,9 +11790,9 @@ async fn test_fold_function_bodies(cx: &mut TestAppContext) {
             fn b() {
                 c();
             }
-
-      -     // this is another uncommitted comment
       -
+      -     // this is another uncommitted comment
+
             fn d() {
                 // e
                 // f