Make 'fsck' able to take an arbitrary number of parents on the

Make 'fsck' able to take an arbitrary number of parents on the
command line.

"arbitrary" is a bit wrong, since it is limited by the argument
size limit (128kB or so), but let's see if anybody ever cares.
Arguably you should prune your tree before you have a few thousand
dangling heads in your archive.

We can fix it by passing in a file listing if we ever care.