revision.c: make default history consider bottom commits

revision.c: make default history consider bottom commits

Previously, the default history treated bottom commits the same as any
other UNINTERESTING commit, which could force it down side branches.

Consider the following history:

   *A--*B---D--*F         * marks !TREESAME parent paths
     \     /*
      `-C-'

When requesting "B..F", B is UNINTERESTING but TREESAME to D. C is
!UNINTERESTING.

So default following would go from D into the irrelevant side branch C
to A, rather than to B.  Note also that if there had been an extra
!UNINTERESTING commit B1 between B and D, it wouldn't have gone down C.

Change the default following to test relevant_commit() instead of
!UNINTERESTING, so it can proceed straight from D to B, thus finishing
the traversal of that path.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>