Clear pending staged/unstaged diff hunks hunks when writing to the git index fails (#26173)

Release Notes:

- Git Beta: Fixed a bug where discarding a hunk in the project diff view
performed two concurrent saves of the buffer.
- Git Beta: Fixed an issue where diff hunks appeared in the wrong state
after failing to write to the git index.

crates/git/src/repository.rs

@@ -862,6 +862,7 @@ pub struct FakeGitRepositoryState {
     pub statuses: HashMap<RepoPath, FileStatus>,
     pub current_branch_name: Option<String>,
     pub branches: HashSet<String>,
+    pub simulated_index_write_error_message: Option<String>,
 }
 
 impl FakeGitRepository {
@@ -881,6 +882,7 @@ impl FakeGitRepositoryState {
             statuses: Default::default(),
             current_branch_name: Default::default(),
             branches: Default::default(),
+            simulated_index_write_error_message: None,
         }
     }
 }
@@ -900,6 +902,9 @@ impl GitRepository for FakeGitRepository {
 
     fn set_index_text(&self, path: &RepoPath, content: Option<String>) -> anyhow::Result<()> {
         let mut state = self.state.lock();
+        if let Some(message) = state.simulated_index_write_error_message.clone() {
+            return Err(anyhow::anyhow!(message));
+        }
         if let Some(content) = content {
             state.index_contents.insert(path.clone(), content);
         } else {