git-cvsexportcommit can't handle merge commits correctly

git-cvsexportcommit can't handle merge commits correctly

git-cvsexportcommit should check if the parent (supplied on the cmdline) to use
for a merge commit is one of the real parents of the merge.

But it errors out if the _first_ parent doesn't match and never checks
the other parents.

Signed-off-by: Peter Baumann <siprbaum@stud.informatik.uni-erlangen.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>