Compare any two revisions (not just adjacent ones)

The revision history diff viewer (overhauled beautifully in \#10824) currently compares each revision against its immediate predecessor. That's great for reviewing incremental changes, but I keep running into situations where I want to compare across a bigger gap, like seeing everything that changed over the course of a week in one view. It'd also be really useful to compare any revision against the *current* content in the editor, not just against another saved revision. The nice thing is that the existing ChangesetHelper.getChangeset() and the Diff extension are not coupled to sequential revisions. So this should be a purely client-side change: 1. Add a way to pick a "compare against" target in the history sidebar (maybe a dropdown defaulting to "Previous revision," or a secondary selection mode) 2. Pass that selection to the changeset helper instead of always using revision.before 3. Show a clear indicator of which two points are being comp…