The most annoying remaining bug is that 3-way merge frequently panics
with "unhandled merge case". This commit fixes that by rewriting the
merge code. The new code is based on the algorithm used in Mercurial
(which was in turn copied from Bazaar):
1. Find "sync" regions, which are regions that are the unchanged in
the base and two sides. Note their start end end positions in each
version.
2. Produce the output by taking the sync regions and inserting the
result of merging the regions between the sync regions. These
regions can either be changed on only one side, in which case we
use that version, or it can be changed on both sides, in which
case we indicate a conflict in the output.
It's both more correct and much easier to follow.
