[migration] don't attempt to carry over values when renaming a computed relation
Without the fix in migractions.py, the introduced test crashes with an
operational error because of an attempt to write to a non-existing
relation table (computed relations are not materialized).
Closes #6304946
usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl