git: Use branch names for resolve conflict buttons (#44421)

This makes merge conflict resolution clearer because we're now parsing
the branch names from the conflict region instead of hardcoding HEAD and
ORIGIN.

### Before
<img width="1157" height="1308" alt="image"
src="https://github.com/user-attachments/assets/1fd72823-4650-48dd-b26a-77c66d21614d"
/>

### After
<img width="1440" height="1249" alt="Screenshot 2025-12-08 at 2 17
12 PM"
src="https://github.com/user-attachments/assets/d23c219a-6128-4e2d-a8bc-3f128aa55272"
/>

Release Notes:

- git: Use branch names for git conflict buttons instead of HEAD and
ORIGIN

crates/git_ui/src/conflict_view.rs

@@ -372,7 +372,7 @@ fn render_conflict_buttons(
         .gap_1()
         .bg(cx.theme().colors().editor_background)
         .child(
-            Button::new("head", "Use HEAD")
+            Button::new("head", format!("Use {}", conflict.ours_branch_name))
                 .label_size(LabelSize::Small)
                 .on_click({
                     let editor = editor.clone();
@@ -392,7 +392,7 @@ fn render_conflict_buttons(
                 }),
         )
         .child(
-            Button::new("origin", "Use Origin")
+            Button::new("origin", format!("Use {}", conflict.theirs_branch_name))
                 .label_size(LabelSize::Small)
                 .on_click({
                     let editor = editor.clone();