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/collab/src/tests/integration_tests.rs

@@ -6770,7 +6770,7 @@ async fn test_remote_git_branches(
 
     assert_eq!(branches_b, branches_set);
 
-    cx_b.update(|cx| repo_b.read(cx).change_branch(new_branch.to_string()))
+    cx_b.update(|cx| repo_b.read(cx).change_branch(new_branch))
         .await
         .unwrap()
         .unwrap();
@@ -6790,23 +6790,15 @@ async fn test_remote_git_branches(
     assert_eq!(host_branch.name, branches[2]);
 
     // Also try creating a new branch
-    cx_b.update(|cx| {
-        repo_b
-            .read(cx)
-            .create_branch("totally-new-branch".to_string())
-    })
-    .await
-    .unwrap()
-    .unwrap();
+    cx_b.update(|cx| repo_b.read(cx).create_branch("totally-new-branch"))
+        .await
+        .unwrap()
+        .unwrap();
 
-    cx_b.update(|cx| {
-        repo_b
-            .read(cx)
-            .change_branch("totally-new-branch".to_string())
-    })
-    .await
-    .unwrap()
-    .unwrap();
+    cx_b.update(|cx| repo_b.read(cx).change_branch("totally-new-branch"))
+        .await
+        .unwrap()
+        .unwrap();
 
     executor.run_until_parked();