merge-base: update the clean-up postprocessing

merge-base: update the clean-up postprocessing

This removes the "contaminate the well even more" approach
taken in the current merge-base postprosessing code.  Instead,
when there are more than one merge-base results, we compute the
merge-base between them and see if one is a fast-forward of the
other, in which case the ancestor is removed from the result.

Signed-off-by: Junio C Hamano <redacted>