git: Add askpass delegate to git-commit handlers (#42239)

In my local setup, I always enforce git-commit signing with GPG/SSH
which automatically enforces `git commit -S` when committing. This
changeset will now show a modal to the user for them to specify the
passphrase (if any) so that they can unlock their private key for
signing when committing in Zed.

<img width="1086" height="948" alt="Screenshot 2025-11-07 at 11 09
09 PM"
src="https://github.com/user-attachments/assets/ac34b427-c833-41c7-b634-8781493f8a5e"
/>


Release Notes:

- Handle automatic git-commit signing by presenting the user with an
askpass modal

crates/fs/src/fake_git_repo.rs

@@ -526,6 +526,7 @@ impl GitRepository for FakeGitRepository {
         _message: gpui::SharedString,
         _name_and_email: Option<(gpui::SharedString, gpui::SharedString)>,
         _options: CommitOptions,
+        _askpass: AskPassDelegate,
         _env: Arc<HashMap<String, String>>,
     ) -> BoxFuture<'_, Result<()>> {
         unimplemented!()