reimplement park

[groenborg]

I want to be able to share a branch in WiP with a coworker.

The park feature is already partially implemented, but was not finished however the few things that was added may need to be rewritten completely. Sync is a part of the park feature and detects if there are conflict by patching in a dry run. Consider a new issue for sync

https://github.com/Praqma/git-phlow/blob/master/phlow/park.go https://github.com/Praqma/git-phlow/blob/master/phlow/sync.go