Renaming a field changes only its own /T, so MASTER.a renamed to b
becomes MASTER.b. set_props compared the bare new partial name with the
other fields' fully qualified names: renaming MASTER.a to b succeeded
while MASTER.b existed, leaving two fields with one fully qualified
name, which are one field with a shared value (ISO 32000-2 §12.7.4.2);
and renaming MASTER.a to c was refused because of an unrelated
top-level field c.
The check now compares the new full name, leaving the field itself out
so a kid keeping its own partial name still succeeds, and the error
names that full name. Top-level renames are checked as before.
Guillem Hermida
fbd9911d76