cvsimport: ignore CVSPS_NO_BRANCH and impossible branches

cvsps output often contains references to CVSPS_NO_BRANCH, commits
that it could not trace to a branch. Ignore that branch.

Additionally, cvsps will sometimes draw circular relationships
between branches -- where two branches are recorded as opening
from the other.  In those cases, and where the ancestor branch
hasn't been seen, ignore it.

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