RFC 277: Able to sync code from Perforce

Created by: unknwon

Make our gitserver able to handle sync request for Perforce depots (relevant section in RFC).

Our gitserver should probably have an interface looks like the following, then implement this interface for both regular Git repositories and Perforce depots.:
```
 type RepositorySyncer interface {
 	Clone(TBD) error
 	Fetch(TBD) error
 }
```
Perforce server authentication is session-based, that means we need to retry a command with p4 login when we get prompted that credential expired. The only small challenge would be p4 login asks for password interactively: https://stackoverflow.com/questions/44471749/golang-enter-ssh-sudo-password-on-prompt-or-exit