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/buffer_diff/Cargo.toml

@@ -17,7 +17,7 @@ test-support = ["settings"]
 [dependencies]
 clock.workspace = true
 futures.workspace = true
-git2.workspace = true
+imara-diff.workspace = true
 gpui.workspace = true
 language.workspace = true
 log.workspace = true