update repo-index to not make two requests in serial #247

this is one of the sources of slowdowns after the switch to XRPC. the appview performs two steps:

• fetches a list of branches
• determines the default ref if any
• fetches the remaining content using the default ref

we can speed this up a lot by doing two things:

• modify all knot endpoints to be agnostic of ref (aka, fallsback to default ref if ref is unspecified), this is already implemented and released with v1.9.1-alpha
• modify repo-index to be agnostic of ref, perform all requests in parallel and then decide how to render the index page based on the responses.