HUGE memory leak somewhere

There seems to be a huge memory leak somewhere, at least on ST3 (haven't tried ST2). I seem to be able to reproduce it simply by opening the history and opening a few files a couple of times, and then simply opening and closing the history panel. You will notice it get slower each time and memory usage sharply increases.
I'm not sure I follow. This code path basically defers the cleaning of history (note the `return` right after this line). I am removing the deferment logic because I don't see the danger in modifying the state maintained in the FileHistory class and/or the FileHistory.json while the overlay is open, as long as `get_history` is returning a copy of the history list (it doesn't even have to be deep).