Overwrite Checker doesn't handle full-attribute overwrites after TPS-style array addition

[apocalyptech]

These two consecutive hotfixes, in TPS, should really trigger an overwrite warning, whereas they currently don't:

set foo bar +(baz)
set foo bar (frotz)

The main issue is that the overwrite checker basically completely discards the TPS array-add syntax, since ordinarily that's completely nondestructive. But discarding it entirely means that it can't pick up on cases like the above.

Note that if this does ever get fixed, the unit tests should probably be exapanded a bit -- for instance, if the set foo bar (frotz) above isn't a hotfix, there wouldn't be any overwrite. We do have a unit test already for this case, which is at the moment expected to fail because of this bug.

Realistically, I kind of doubt this'll ever get fixed, since that syntax is hardly used, and I don't particularly feel like digging through the overwrite code sufficiently to implement a change like this. But, I suppose we'll see.