cmd-index: eliminate redundant paths from index update command.

cmd-index: eliminate redundant paths from index update command.

If someone asks to update "/etc" and "/etc/passwd", the latter is redundant
because it's included in the first.  Don't bother updating the file twice
(and thus causing two index merges, etc).

Ideally we would only do one merge for *any* number of updates (etc /etc and
/var).  This should be possible as long as we sort the entries correctly
(/var/ and then /etc/), since a single sequential indexfile could just have
one appended to the other.  But we don't do that yet.