Don't let Detect() bottom out to checking //

NOTE: I'm not a an experienced vim coder so please make this change yourself if adding this hacky if is not the proper way :)

Solves bug for cygwin users where this would cause a network lookup which can be painfully slow in windows.

This is the same bug as: https://github.com/tpope/vim-fugitive/issues/210

On windows if you end up checking a '//...' path for anything windows decides the best course of action is to stall out and check network paths.