Scrolling up to the top of a log should cause more history to be loaded

[mnutt]

Right now when you switch from one process log to another we load in the last 1000 lines of history. This is sort of a compromise since the more history we load the slower the window is to initially render, but it's nice to be able to go back to earlier history.

We currently have hooks for scrolling in a log window and a mechanism for using tail to fetch logs; we would just need to figure out the correct offsets and then prepend the lines to the beginning of the buffer. Ideally as the user scrolled up more history would be filled in smoothly, rather than jumping all the way to the top as new history was added.