Don't re-search not-matching notes when search text appended to

[seanh]

If the user has just added a character to the end of the search text, then any notes that didn't match before can't possibly match now, so there's no point in search them, just search the matching ones. And if there were none matching before, there will still be none, so don't search at all.

Seems like a big optimisation and an easy one,