cvsclient: handle files which lack a trailing newline

CVS uses \n as a protocol message separator, which forces us
to read protocol message line-by-line. File content sent by
the server has a length known and is transmitted in bytes.
The server appends a final "ok\n" message (or perhaps an error
message) when it is done sending file contents.

Properly handle the case where this final message gets buffered
along with file contents and is not delimited from file contents
by \n because the file lacks a trailing newline. Previously, the
final protocol message ended up being written out to file contents
in this case.

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