Added pasthological 'oldhead' repo that produces a content mismatch.

All regressiion tests pass.
Simplifty the oldhead repository by removing a revision.
Add TARGET_ARCH to Makefile link invocation.
Change a statistics counter variable to a double.

This will give integer semantics upto about 8 Petabytes.
After that you may get rounding errors ...
Speed up merge_branches

Maintain a sort buffer of the revisions in date order
for ease of finding cliques. After an initial sort, try
and minimize future sort ops.

The easy way of just sorting the revisions array works
as far as speeding up merge goes, but it kills the fast
out performance (it breaks the assumptions of PRESORT)

the naive sorting method cut the netbsd-pkgsrc merge from
~3300s to ~1700s. With the additional machinery to avoid
killing fast out, it is more modest. down to ~2900s

On emacs historic, merge went down from 21s to 16s