t7400: add !CYGWIN prerequisite to 'add with \\ in path'

Commit cf9e55f494 ("submodule: prevent backslash expantion in submodule names", 07-04-2017) added a test which creates a git repository with some backslash characters in the name. On windows, where the backslash character is a directory separator, it is not possible to create a repository with the name 'sub\with\backslash'. (The NTFS filesystem would probably allow it, but the win32 api does not). The MinGW and Git for Windows versions of git actually create a repository called 'backslash' in the sub-directory 'sub/with'. On cygwin, however, due to the slightly schizophrenic treatment of the backslash character by cygwin-git, this test fails at the 'git init' stage. The git-init command does not recognise the directory separators in the input path (eg. is_dir_sep('\\') is false), so it does not attempt to create the leading directories 'sub/with'. (The call to mkdir('sub\\with\\backslash') actually does recognise the directory separators, but fails because the 'sub/with' directory doesn't exist). In order to suppress the test failure (for now), add the !CYGWIN test prerequisite. Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>