Subversion, compare with branch start revision

This is with IDEA 9.01.

I'm looking for how to easily compare my Subversion branch with the point in the trunk, that is the revision, from which the branch started.

I can do this by choosing Subversion, Compare With... and then selecting the revision, but then I have to keep selecting this revision on each compare. I'd like to be able, at any time, to ask, "what have I changed since I branched?".

Note this is not the same thing as comparing, say, with trunk, which I can do using Compare with Branch.