Support cross-repository branch reference

[RoKish]

Description

Currently, the action always namespace a branch name with a repository owner name (repo-owner:branch-name). Because of this, it's impossible to find a pull request created from a forked repository. This pull request changes this behaviour. The action will append the owner prefix only in the case branch name doesn't already have one (i.e. it doesn't contain :).

[juliangruber]

What about instead of parsing the string, we add another input(s) for this?

[RoKish]

Yeah, we can do that, but I don't really see any problems with using just one parameter. Branch names are not allowed to contain colons (:), so the current approach is safe and backward compatible. Moreover, I find this approach more user-friendly since the configuration is easier:

1. The user doesn't need to look into the documentation to realize the difference between the parameters
2. There's no need to define priority, i.e. what to do if two parameters are specified.

[juliangruber]

Thanks for explaining! https://github.com/juliangruber/find-pull-request-action/releases/tag/v1.3.0