limiting git branch --contains

[Oswald Buddenhagen <oswald.buddenhagen@gmx.de>]

moin,

git branch --contains can be a rather expensive operation in big 
repositories. as my use case is actually a rather limited search for 
commits in my local wip branches, it would be helpful to be able to 
specify exclusions for the rev-walk, say

   git branch --contains deadbeef ^origin/master

suggestions how this is actually already achievable efficiently are of 
course welcome as well. ^^

thanks!