| One way to do this would be to first of all delete /trunk
| from the repository and rebuild it from the current contents
| of axiom--silver--1, but given the little I understand about
| how SVN works, I am concerned that using svnadmin to delete
| /trunk might have serious consequences for any branch cloned
| from /trunk (such as build-improvements). Am I wrong about
| that?
I don't know all the gory details of SVN implementation, but my
understanding is since SVN works by changeset, any change you make to
the repository is recorded as a global change with a given revision
number (even those branches that are not directly affected see the
change in the revision number). Consequently, if you "delete" trunk
-- but not the metadata -- and replace it with a new copy, then
technically SVN would record all those changes, and the diff between
the branches should be able to compute correctly the change set. This
assumes the "delete-and-replace" operation is trouble-free.
This is obvious, but please make a backup of the repo before you start :-)