cindex: fix sorting and uniqueness

We can't rely on combining the `-u' and `-k1,1' switches of POSIX
sort(1) to do what we want.  So only rely on `sort -k1,1' while
introducing a small Perl helper to fold identical prefixes into
one line.  In other words, input such as:

  deadbeef 0
  deadbeef 1
  deadbeef 2

Was getting deduplicated into a single line:

  deadbeef 0

... with `sort -u -k1,1'
This makes puts the output into a more optimal form for eventual
(not-fully-implemented-yet) parsing:

  deadbeef 0,1,2

ORS is current the comma (`,') for inbox IDs, but it'll be a
space (` ') for coderepo root IDs.  This implementation also
combines identical IDs in the 2nd column.  Thus:

  deadbeef 0
  deadbeef 0

Becomes a single `deadbeef 0' line thanks to the use of
XS List::Util::uniq (which beats a pure Perl hash).

I attempted to implement this in awk but Perl is close enough to
gawk in performance while being shorter and easier-to-understand
due to List::Util::uniq.  mawk was faster, but still not enough
to matter as the bottleneck is from iterating through Xapian
MSets.

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