git-show-branch: tighten merge-base computation.
git-show-branch: tighten merge-base computation.

This makes the merge-base computation resistant to the pathological
case discussed on the list earlier, by doing the same logic as
git-merge-base.  As a side effect, it breaks the command's primary
function to list non-merge commit sequences, which needs to be fixed
separately.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git