`svn diff' after `svn merge' that adds a file fails

suppose that a trunk and branch are identical. i add a couple of files
to the trunk and check the changes in. then i try to use `svn merge'
to apply the changes in the trunk to the changes in the branch.

the merge command completes successfully and adds the files like you
think it would. svn status lists the files as added with history:

A  +   po/pt.gmo
A  +   po/pt.po
A  +   themus.spec

however, running `svn diff' produces a diff of any files that were
changed, but when it gets to a file that was just added, like
themus.spec, it says:

svn: No such file or directory
svn: svn_io_file_open: can't open `.svn/text-base/themus.spec.svn-base'

also, i created a test repository and checked out a copy of the root
of the repository. i added a file using `svn add' and then checked
.svn/test-base to see if it had a .svn-base file and it didnt.
however, the `svn diff' command correctly show all of the lines of the
file as being added.

Original issue reported by graham