cmd-index: only update if -u is given; print only given file/dirnames.

cmd-index: only update if -u is given; print only given file/dirnames.

cmd-index now does two things:
 - it updates the index with the given names if -u is given
 - it prints the index if -p, -s, or -m are given.

In both cases, if filenames are given, it operates (recursively) on the
given filenames or directories.  If no filenames are given, -u fails (we
don't want to default to /; it's too slow) but -p/s/m just prints the whole
index.