cygwin: disallow backslashes in file names

The backslash character is not a valid part of a file name on Windows.
If, in Windows, Git attempts to write a file that has a backslash
character in the filename, it will be incorrectly interpreted as a
directory separator.

This caused CVE-2019-1354 in MinGW, as this behaviour can be manipulated
to cause the checkout to write to files it ought not write to, such as
adding code to the .git/hooks directory.  This was fixed by e1d911dd4c
(mingw: disallow backslash characters in tree objects' file names,
2019-09-12).  However, the vulnerability also exists in Cygwin: while
Cygwin mostly provides a POSIX-like path system, it will still interpret
a backslash as a directory separator.

To avoid this vulnerability, CVE-2021-29468, extend the previous fix to
also apply to Cygwin.

Similarly, extend the test case added by the previous version of the
commit.  The test suite doesn't have an easy way to say "run this test
if in MinGW or Cygwin", so add a new test prerequisite that covers both.

As well as checking behaviour in the presence of paths containing
backslashes, the existing test also checks behaviour in the presence of
paths that differ only by the presence of a trailing ".".  MinGW follows
normal Windows application behaviour and treats them as the same path,
but Cygwin more closely emulates *nix systems (at the expense of
compatibility with native Windows applications) and will create and
distinguish between such paths.  Gate the relevant bit of that test
accordingly.

Reported-by: RyotaK <security@ryotak.me>
Helped-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
