Fix test migration crash waiting to happen due to inferred relations
Avoid crashing when a relation definition which is already known (due to
being inferred) gets explicitly added. As explained in a comment, this
would deserve more thinking, but at least this fixes a test failure with
computed relations.
msgid ""
msgstr ""
"MIME-Version: 1.0\n"
"Content-Type: text/plain; charset=UTF-8\n"
"Content-Transfer-Encoding: 8bit\n"
"Generated-By: pygettext.py 1.5\n"
"Plural-Forms: nplurals=2; plural=(n > 1);\n"
"PO-Revision-Date: YEAR-MO-DA HO:MI +ZONE\n"