Here's the comment in the code:
/* Copying from transaction roots not currently available.
### cmpilato todo someday: make this not so. :-) Note that
when copying from mutable trees, you have to make sure that
you aren't creating a cyclic graph filesystem, and a simple
referencing operation won't cut it. Currently, we should not
be able to reach this clause, and the interface reports that
this only works from immutable trees anyway, but JimB has
stated that this requirement need not be necessary in the
future. */
Task
Minor
SVN-790 Allow transaction roots as copy sources in the filesystem.
