How to do a reverse rebase?

Okay, this is picayune whining, but when you've fixed all the big
bugs...

I don't want to rebase HEAD on *that*, but rather rebase *that*
on top of the current HEAD.

Sometimes I have a little debug hack on a branch by itself, and I
discover that I need it again, so I want to rebase it on top of
current development.

But there's been a LOT of development in the meantime.  And if I do

git-rebase HEAD debug_hack

git first checks out debug_hack.  This takes a while and, more
importantly, every file modified in HEAD...debug_hack has its timestamp
touched and make(1) insists on recompiling it.

I want to only modify the three files that are touched on the debug_hack
branch, so my recompile times aren't too long.

Currently, when I remember, I'll use git-cherry-pick and manually
rename branches.

Is there an easier way?  Or should I just learn StGit?