Fix helm-projectile-vc

Currently we cannot switch to a project using vc-dir or magit-status
because helm-projectile-vc cannot pass the selected project directory
into projectile-vc. This fix makes it simpler to switch to VC status
anywhere without depending on project root. helm-projectile needs its
own helm-projectile-vc command.

The change to projectile-project-vc also makes it much faster to switch
project because it does not need to compute project root when explicitly
supplied. Otherwise, there's a brief unnecessary delay.