git-svn: exclude already merged tips using one rev-list call

git-svn: exclude already merged tips using one rev-list call

The old function would have to check all mentioned merge tips, every time
that the mergeinfo ticket changed.  This involved 1-2 rev-list operation
for each listed mergeinfo line.  If there are a lot of feature branches
being merged into a trunk, this makes for a very expensive operation for
detecting the new parents on every merge.

This new version first uses a single 'rev-list' to figure out which commit
ranges are already reachable from the parents.  This is used to eliminate
the already merged branches from the list.

Signed-off-by: Sam Vilain <sam@vilain.net>
Acked-by: Eric Wong <normalperson@yhbt.net>