determine branch's ancestor?

[Tyler]

I have been negligent in my record-keeping and find that i don't know
from what branch i cut a new branch. That is, did i cut feature branch
FEATURE from the trunk, or from release branch RELEASE?

Most of the work on FEATURE touches new files or files that haven't
changed in months, so the branch tags RELEASE and FEATURE both point to
the same versions of the files (the last HEAD revision), so i can't use
that to help me determine the ancestor.

How else can i do this?

tyler