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

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>