Diffing on undo steps is a critical performance point of override
system, although not required for override itself, it gives user
immediate feedback ove what is overridden.
Profiling showed that rna path text search over overrides operations was
by far the most costly thing here, so now using a runtime temp ghash
mapping for this search instead.
Seems to give at least 5 times speedup on big production rig.
Bastien Montagne
2a38b857f7