When calling loadBackup with no backup branch found, ask user for a backup branch name

[maxyu1115]

When loadBackup is called on a branch where the extension can't detect a backup branch, ask the user if they want to loadBackup by giving a backup branch name.

This is useful when the user doesn't want to commit their .bbinfo onto their branch, but on a new computer, want to load the backup. With this feature, on their new computer, users can pull the branch they want to loadBackup, then loadBackup by manually typing the backupBranch's name.

[maxyu1115]

I'm still not 100% sure this is the best way to address this problem with not wanting to commit their .bbinfo file. Open for discussion