Don't fetch every branch in contributor's remote when checking out a pull request

Thanks for this very nice extension!! It is really nice to review code in my code editor:)

I noticed that when I clicked the blue "Checkout" button in the pull request tab, the following happens:

1. The extension adds a remote named after the GitHub username of the pull-request author (this remote is not deleted when I "Exit review mode")
2. This new remote is fetched (this creates remote-tracking branches for all branches in the contributor's remote)
3. The the source branch of the pull request is checked out (this branch is not deleted when I "Exit review mode")

I don't like 2), it pollutes the output of git branch -r.
I think the extension should only fetch the branch of the pull request, i.e.

git fetch $PR_AUTHOR_REMOTE_URL $PR_BRANCH:refs/heads/pr/$AUTHOR_GITHUB_USERNAME/$PR_NUMBER

In fact, adding a remote is not even necessary as Git can fetch a branch directly into FETCH_HEAD, and we can the checkout FETCH_HEADin detached HEAD.

I understand the current behaviour might be convenient for some, but maybe it could be configurable ?

Bắt đầu từ luồng Checkout của tab pull request và theo dõi cách luồng này thêm contributor remote, fetch remote đó rồi checkout source branch. So sánh hành vi này với branch-only fetch và detached checkout được đề xuất trong issue, sau đó xác định các test hiện có cho checkout ở chế độ review; hoàn thành khi các contributor branch không liên quan không còn xuất hiện trong git branch -r và việc cleanup vẫn hoạt động.