difftool: Disable --symlinks on cygwin

Symlinks are not ubiquitous on Windows so make --no-symlinks the default.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -338,7 +338,8 @@ sub main
 		gui => undef,
 		help => undef,
 		prompt => undef,
-		symlinks => $^O ne 'MSWin32' && $^O ne 'msys',
+		symlinks => $^O ne 'cygwin' &&
+				$^O ne 'MSWin32' && $^O ne 'msys',
 		tool_help => undef,
 	);
 	GetOptions('g|gui!' => \$opts{gui},