Feature request: fork only when necessary

[bienzaaron]

The feature request
I would like the ability to fork a repository, only if I don't have access to push to the target branch on the main repository.

I think this could be accomplished by adding a --fork-if-necessary flag, or alternatively by adding additional behavior to fork like: --fork=necessary | --fork=always, where --fork implies --fork=always.

My use case
Some checks exists on certain repos of mine (which are out of my control) where certain files can only be modified by a PR from the main repo. This would allow me to push to the main repo wherever possible and satisfy those checks.

[bienzaaron]

I am open to contributing this feature if it makes sense to add.