A checkpoint commit; this adds a first stab attempt at a
global (across all files) hueristic for determining the
tag<->branchtag mapping. It seem to be working, but has not
been deeply tested yet at all. There is also debugging cruft
that should be removed before this goes into a release.
Also, ALL of the big (per-file or more) hashes now get stored
as tie'ed databases, in order to deal with memory exhaustion
when dealing with BIG data sets.