html-diff: Also update index-pane hashes when changed

The buttons in the top left have their hashes (which choose which mode the next part opens in) changed when the current mode changes. This commit adds the same for the links on the right - currently the mode is reset if you choose any of them.

This might be better to be a localStorage thing along with things like layer sets and index pane visibility, but for now this is a simple change and fixes something that trips me up fairly often.