Implement staging of partially-staged hunks (#25520)

Closes: #25475 

This PR makes it possible to stage uncommitted hunks that overlap but do
not coincide with an unstaged hunk.

Release Notes:

- Made it possible to stage hunks that are already partially staged

---------

Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>
Co-authored-by: Max <max@zed.dev>

crates/editor/src/test/editor_test_context.rs

@@ -285,7 +285,7 @@ impl EditorTestContext {
         snapshot.anchor_before(ranges[0].start)..snapshot.anchor_after(ranges[0].end)
     }
 
-    pub fn set_diff_base(&mut self, diff_base: &str) {
+    pub fn set_head_text(&mut self, diff_base: &str) {
         self.cx.run_until_parked();
         let fs = self.update_editor(|editor, _, cx| {
             editor.project.as_ref().unwrap().read(cx).fs().as_fake()
@@ -298,6 +298,19 @@ impl EditorTestContext {
         self.cx.run_until_parked();
     }
 
+    pub fn set_index_text(&mut self, diff_base: &str) {
+        self.cx.run_until_parked();
+        let fs = self.update_editor(|editor, _, cx| {
+            editor.project.as_ref().unwrap().read(cx).fs().as_fake()
+        });
+        let path = self.update_buffer(|buffer, _| buffer.file().unwrap().path().clone());
+        fs.set_index_for_repo(
+            &Self::root_path().join(".git"),
+            &[(path.into(), diff_base.to_string())],
+        );
+        self.cx.run_until_parked();
+    }
+
     #[track_caller]
     pub fn assert_index_text(&mut self, expected: Option<&str>) {
         let fs = self.update_editor(|editor, _, cx| {