The old function was incorrect; in some instances it marks a cherry picked
range as a merged branch (because of an incorrect assumption that
'rev-list COMMIT --not RANGE' would work). This is replaced with a
function which should detect them correctly, memoized to limit the expense
of dealing with branches with many cherry picks to one 'merge-base' call
per merge, per branch which used cherry picking.
Signed-off-by: Sam Vilain <sam@vilain.net> Acked-by: Eric Wong <normalperson@yhbt.net>