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>

t/t5580-clone-push-unc.sh

@@ -3,12 +3,18 @@
 test_description='various Windows-only path tests'
 . ./test-lib.sh
 
-if ! test_have_prereq MINGW; then
+if test_have_prereq CYGWIN
+then
+	alias winpwd='cygpath -aw .'
+elif test_have_prereq MINGW
+then
+	alias winpwd=pwd
+else
 	skip_all='skipping Windows-only path tests'
 	test_done
 fi
 
-UNCPATH="$(pwd)"
+UNCPATH="$(winpwd)"
 case "$UNCPATH" in
 [A-Z]:*)
 	# Use administrative share e.g. \\localhost\C$\git-sdk-64\usr\src\git
@@ -45,8 +51,8 @@ test_expect_success push '
 	test "$rev" = "$(git rev-parse --verify refs/heads/to-push)"
 '
 
-test_expect_success 'remote nick cannot contain backslashes' '
-	BACKSLASHED="$(pwd | tr / \\\\)" &&
+test_expect_success MINGW 'remote nick cannot contain backslashes' '
+	BACKSLASHED="$(winpwd | tr / \\\\)" &&
 	git ls-remote "$BACKSLASHED" >out 2>err &&
 	test_i18ngrep ! "unable to access" err
 '