Merge branch 'tb/push-to-cygwin-unc-path' into maint

Merge branch 'tb/push-to-cygwin-unc-path' into maint

On Cygwin, similar to Windows, "git push //server/share/repository"
ought to mean a repository on a network share that can be accessed
locally, but this did not work correctly due to stripping the double
slashes at the beginning.

This may need to be heavily tested before it gets unleashed to the
wild, as the change is at a fairly low-level code and would affect
not just the code to decide if the push destination is local.  There
may be unexpected fallouts in the path normalization.

* tb/push-to-cygwin-unc-path:
  cygwin: allow pushing to UNC paths