History mode efficiency

[edemaine]

• Should remember the diff sequence when leaving and re-entering history view for the same page. Saves client and server and network. Clear when loading for a different page. Load any diffs since the past diff when re-entering.
• Then can remember what diff you were on, so when re-entering, jump to the remembered location in time. This should make it more convenient to jump back and forth between past and present.
• Might also want to limit the number of returned diffs at once, and require client to do a bunch of fetches, to limit the cost of each method call.
• Should also compute reverse diffs when applying a diff (like we do for undo stack -- maybe we can even use that data structure?). Then we can scrub backwards as quickly as we can forwards.