cvsclient: handle additional responses sent by server

While checking out files the server sends messages to the CVS
client which provide information about the state of file paths.

Our custom CVS client implementation needs to recognize a few
additional responses the server may send while checking out a
different version of a file which was already checked earlier.
Otherwise our client will error out. We can simply ignore the
messages (and its 2 paths arguments separated by \n) because
we do not manage an actual CVS working copy.

Found while testing ingestion of the GNU dino repository at
cvs.savannah.gnu.org/sources/dino