This follows up on changeset 134521c47201 which introduced the extra
check about the absence of 'data_name' in cw_edited. However, when
coming from a web form (at least as of today's behavior of cubicweb),
this field is always present leading to the if: block never being
executed.

We hence change it to only avoid override of 'data_name' if is explicitly
changed (i.e. when the "old" and "new" values differ).

Test (test_file_name_priority) introduced in aforementioned changesets
still passes.