Add support for `pr checkout` of origin branches

[majuscule]

When working with a team and submitting branches for code review that have been pushed to the master repository, pr checkout fails to properly fetch from origin/ prefaced branches. This fixes that.

I avoided using the s//r non-destructive regex modifier to retain support for older perls.

[moollaza]

+1 would love to see this merged -- solves a problem I run into daily!