`build-aux/git-version-gen` always requires $1 even in a git repo

[Cedric Sun]

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