t5580: add Cygwin support

t5580: add Cygwin support

t5580 tests that specifying Windows UNC paths works with Git.  Cygwin
supports UNC paths, albeit only using forward slashes, not backslashes,
so run the compatible tests on Cygwin as well as MinGW.

The only complication is Cygwin's `pwd`, which returns a *nix-style
path, and that's not suitable for calculating the UNC path to the
current directory.  Instead use Cygwin's `cygpath` utility to get the
Windows-style path.

Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>