Hi,
According to line #21 comment in that file, niladic invocation of the script should work in a git repo without a `.tarball-version` file.
The behavior is currently different, and requires $1 argument to be provided even inside a git repo. When that's the case, any dummy $1 works:
cd gnulib-git
./build-aux/git-version-gen # $usage printed
./build-aux/git-version-gen foobar # `0.1.5512-15498`
I'm not sure if this is by design (for backward compatibility?) or a bug.
Regards,
-vc