diff A...B: do not limit the syntax too narrowly

diff A...B: do not limit the syntax too narrowly

Earlier we tried to make sure that the trees we get are what A...B
syntax produced, by checking that earlier ones are all marked
uninteresting (which has to be true as they are merge bases),
there are two remaining ones that are interesting, and they are
marked as non-symmetric-left and symmetric-left respectively.

The "the last two must be interesting" condition is however wrong when one
is an ancestor of the other between A and B (i.e. fast-forward).  In such
a case, one of them is marked uninteresting.