When merging branches based on svk:merge properties, a single merge
can have updated or added multiple svk:merge lines. Attempt to
include the minimal set of parents by sorting the merge properties in
order of revision, highest to lowest.
Signed-off-by: Alex Vandiver <alex@chmrr.net> Acked-by: Sam Vilain <sam@vilain.net> Acked-by: Eric Wong <normalperson@yhbt.net>