t1301-*.sh: Fix the 'forced modes' test on cygwin

t1301-*.sh: Fix the 'forced modes' test on cygwin

The 'forced modes' test fails on cygwin because the post-update
hook loses it's executable bit when copied from the templates
directory by git-init. The template loses it's executable bit
because the lstat() function resolves to the "native Win32 API"
implementation.

This call to lstat() happens after git-init has set the "git_dir"
(so has_git_dir() returns true), but before the configuration has
been fully initialised. At this point git_config() does not find
any config files to parse and returns 0. Unfortunately, the code
used to determine the cygwin l/stat() function bindings did not
check the return from git_config() and assumed that the config
was complete and accessible once "git_dir" was set.

In order to fix the test, we simply change the binding code to
test the return value from git_config(), to ensure that it actually
had config values to read, before determining the requested binding.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>