merge-recur: try to merge older merge bases first

merge-recur: try to merge older merge bases first

It seems to be the only sane way to do it: when a two-head merge is
done, and the merge-base and one of the two branches agree, the
merge assumes that the other branch has something new.

If we start creating virtual commits from newer merge-bases, and go
back to older merge-bases, and then merge with newer commits again,
chances are that a patch is lost, _because_ the merge-base and the
head agree on it. Unlikely, yes, but it happened to me.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>