Don't suggest chmod +/-x on Windows

Some of the executable error messages suggest using chmod +/-x to toggle the executable bits. This works fine on *nix, but not on windows, and the results range from 'command not found' to doing nothing at all.

Eg: https://github.com/pre-commit/pre-commit-hooks/blob/master/pre_commit_hooks/check_executables_have_shebangs.py#L66

Instead, this should recommend git add --chmod=+/-x somepath when the hook detects it's running on windows.