1.5.0.rc1.gf4b6c: git-cvsimport: Some nits

Running cvsimport with bogus arguments gives an usage message including -a,
which is gone (at least the latest man page doesn't mention it anymore).

Besides, running git-cvsimport in something that isn't a repo gives the
(useless) error message:

  CVSROOT needs to be set at /home/vonbrand/bin/git-cvsimport line 105.
-- 
Dr. Horst H. von Brand                   User #22616 counter.li.org
Departamento de Informatica                    Fono: +56 32 2654431
Universidad Tecnica Federico Santa Maria             +56 32 2654239
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