This change introduces a new PromiseQueue, which replaces our old

$.funcQueue system. This works very similarly, where we can add a

bunch of items to the queue and then start it, potentially adding items

even as its running. The big difference is that the individual functions

don't need to care at all that they're running within the queue or ask

the queue to move on to the next item, because they're just promises or

async functions.
This also adds an abort signal, which is both handled internally in the

queue and also passed through to any functions that are included. This

fixes one of our long-standing bugs where rapid switching between

interdiff revisions or commit ranges would cause the displayed diffs to

be incorrect, showing files multiple times or diffs from the wrong

revisions.

Ran js-tests.
Loaded a ton of different diffs and switched rapidly between revisions

  and commit ranges. Saw that the correct files were always loaded and

  we never got weird duplicates.