How to find where a branch was taken from.

I need a command that will find the remote branch from which
the currently checked out branch was started. I don't know
git very well, and the only way I can think to do it so far
is to iterate over the remote branches and find the one
for which git-rev-list <branch>..HEAD gives the smallest
number of objects. I'm guessing there must be a better
way. Any ideas?

Cheers,
	Paul.