R1:904dbf0db666

Make the "git upload-pack" proxy more robust

Summary:
Depends on D20381. Ref T8093. This makes minor improvements to the protocol proxy to handle cases where we add, remove, or replace refs and may need to move the "capabilities" section.

Rather than invoking a callback on every ref: parse the whole ref list into a data structure, mutate it if necessary (in a future diff), then dump it back into wire format.

This allows us to shift the capabilities data (which needs to be coupled with the first ref)…