cvsimport: complete the cvsps run before starting the import

We now capture the output of cvsps to a tempfile, and then read it in.
cvsps 2.1 works quite a bit "in memory", and only prints its patchset
info once it has finished talking with cvs, but apparently retaining
all that memory allocation. With this patch, cvsps is finished and
reaped before cvsimport start working (and growing). So the footprint
of the whole process is much lower.

Signed-off-by: Martin Langhoff <martin@catalyst.net.nz>
Signed-off-by: Junio C Hamano <junkio@cox.net>