fixed: commit/git: file named 'nil' is created everytime when get Rake::Delphi::Git.version

There is an logic to evaluate NULL device to hide an error message
if `git describe` is run outside of git repository:
'/dev/null' for Cygwin and 'nil' otherwise (it's not intended to be run
on Linux). By mistake that name is 'nil' but it must be 'nul'.