git_ui: Branch picker improvements (#26287)

- Truncate branch names based on the width of the picker
- Use a footer for "Create branch" instead of a picker entry

Still to do:

- [x] Select the footer button when no matches and run the create logic
on `enter`
- [x] Make it possible to quickly select the footer button from the
keyboard when there are matches

Release Notes:

- Git Beta: Removed limitation that made it impossible to create a
branch from the branch picker when it too closely resembled an existing
branch name

crates/remote_server/src/remote_editing_tests.rs

@@ -1361,7 +1361,7 @@ async fn test_remote_git_branches(cx: &mut TestAppContext, server_cx: &mut TestA
 
     assert_eq!(&remote_branches, &branches_set);
 
-    cx.update(|cx| repository.read(cx).change_branch(new_branch.to_string()))
+    cx.update(|cx| repository.read(cx).change_branch(new_branch))
         .await
         .unwrap()
         .unwrap();
@@ -1383,23 +1383,15 @@ async fn test_remote_git_branches(cx: &mut TestAppContext, server_cx: &mut TestA
     assert_eq!(server_branch.name, branches[2]);
 
     // Also try creating a new branch
-    cx.update(|cx| {
-        repository
-            .read(cx)
-            .create_branch("totally-new-branch".to_string())
-    })
-    .await
-    .unwrap()
-    .unwrap();
+    cx.update(|cx| repository.read(cx).create_branch("totally-new-branch"))
+        .await
+        .unwrap()
+        .unwrap();
 
-    cx.update(|cx| {
-        repository
-            .read(cx)
-            .change_branch("totally-new-branch".to_string())
-    })
-    .await
-    .unwrap()
-    .unwrap();
+    cx.update(|cx| repository.read(cx).change_branch("totally-new-branch"))
+        .await
+        .unwrap()
+        .unwrap();
 
     cx.run_until_parked();