Implement staging and unstaging hunks (#24606)

- [x] Staging hunks
- [x] Unstaging hunks
- [x] Write a randomized test
- [x] Get test passing
- [x] Fix existing bug in diff_base_byte_range computation
- [x] Remote project support
- [ ] ~~Improve performance of
buffer_range_to_unchanged_diff_base_range~~
- [ ] ~~Bug: project diff editor scrolls to top when staging/unstaging
hunk~~ existing issue
- [ ] ~~UI~~ deferred
- [x] Tricky cases
  - [x] Correctly handle acting on multiple hunks for a single file
- [x] Remove path from index when unstaging the last staged hunk, if
it's absent from HEAD, or staging the only hunk, if it's deleted in the
working copy

Release Notes:

- Add `ToggleStagedSelectedDiffHunks` action for staging and unstaging
individual diff hunks

crates/git/src/repository.rs

@@ -8,6 +8,8 @@ use gpui::SharedString;
 use parking_lot::Mutex;
 use rope::Rope;
 use std::borrow::Borrow;
+use std::io::Write as _;
+use std::process::Stdio;
 use std::sync::LazyLock;
 use std::{
     cmp::Ordering,
@@ -39,6 +41,8 @@ pub trait GitRepository: Send + Sync {
     /// Note that for symlink entries, this will return the contents of the symlink, not the target.
     fn load_committed_text(&self, path: &RepoPath) -> Option<String>;
 
+    fn set_index_text(&self, path: &RepoPath, content: Option<String>) -> anyhow::Result<()>;
+
     /// Returns the URL of the remote with the given name.
     fn remote_url(&self, name: &str) -> Option<String>;
     fn branch_name(&self) -> Option<String>;
@@ -161,6 +165,50 @@ impl GitRepository for RealGitRepository {
         Some(content)
     }
 
+    fn set_index_text(&self, path: &RepoPath, content: Option<String>) -> anyhow::Result<()> {
+        let working_directory = self
+            .repository
+            .lock()
+            .workdir()
+            .context("failed to read git work directory")?
+            .to_path_buf();
+        if let Some(content) = content {
+            let mut child = new_std_command(&self.git_binary_path)
+                .current_dir(&working_directory)
+                .args(["hash-object", "-w", "--stdin"])
+                .stdin(Stdio::piped())
+                .stdout(Stdio::piped())
+                .spawn()?;
+            child.stdin.take().unwrap().write_all(content.as_bytes())?;
+            let output = child.wait_with_output()?.stdout;
+            let sha = String::from_utf8(output)?;
+
+            log::debug!("indexing SHA: {sha}, path {path:?}");
+
+            let status = new_std_command(&self.git_binary_path)
+                .current_dir(&working_directory)
+                .args(["update-index", "--add", "--cacheinfo", "100644", &sha])
+                .arg(path.as_ref())
+                .status()?;
+
+            if !status.success() {
+                return Err(anyhow!("Failed to add to index: {status:?}"));
+            }
+        } else {
+            let status = new_std_command(&self.git_binary_path)
+                .current_dir(&working_directory)
+                .args(["update-index", "--force-remove"])
+                .arg(path.as_ref())
+                .status()?;
+
+            if !status.success() {
+                return Err(anyhow!("Failed to remove from index: {status:?}"));
+            }
+        }
+
+        Ok(())
+    }
+
     fn remote_url(&self, name: &str) -> Option<String> {
         let repo = self.repository.lock();
         let remote = repo.find_remote(name).ok()?;
@@ -412,6 +460,20 @@ impl GitRepository for FakeGitRepository {
         state.head_contents.get(path.as_ref()).cloned()
     }
 
+    fn set_index_text(&self, path: &RepoPath, content: Option<String>) -> anyhow::Result<()> {
+        let mut state = self.state.lock();
+        if let Some(content) = content {
+            state.index_contents.insert(path.clone(), content);
+        } else {
+            state.index_contents.remove(path);
+        }
+        state
+            .event_emitter
+            .try_send(state.path.clone())
+            .expect("Dropped repo change event");
+        Ok(())
+    }
+
     fn remote_url(&self, _name: &str) -> Option<String> {
         None
     }