Default refspec for branches

Is there a way to change the default refspec that git pull uses on a
per branch basis?  I know that you can create .git/remotes/<foo> and
do 'git pull <foo>' to pull from whatever is listed in there, but that
isn't quite what I'm looking for.

What I'd like to be able to do is create a branch and have 'git pull'
simply pull from the remote tree.  I tried listing multiple refspecs
in .git/remotes/origin, but git didn't seem to like that.

For example, I clone Linus' tree.  Then I create a branch called mtd. 
When I'm working in that branch, I want 'git pull' to pull from
git://git.infradead.org/mtd-2.6.git.

Any ideas?

josh