changelog merge hook should use a 3-way merge

Currently the debian/changelog merge hook only parses THIS and OTHER to compute the merge. It should also look at BASE. This would enable a few things.

1) It should conflict if THIS and OTHER both change the same section. (Or possibly try to do a regular 3-way merge on the internal lines?)
2) It should be able to decide whether THIS or OTHER simply updated a section, and thus just use that section.
3) We may want to change the code so that for sections where THIS == OTHER == BASE we don't parse them at all. (It may be tricky to find the sections, though.)