We do already have `http-headers`, which potentially could be used to
allow clients to completely override the field, but it seems like the
more common use case is simply to append.
Closes: #1496
Approved by: cgwalters
Allow users to pass `<remote>:` to list all refs we have locally
belonging to `<remote>`. Also (re-)allow the similar `<remote>:.` syntax
for backwards compatibility with flatpak.
Closes: #1500
Approved by: cgwalters
Jonathan Lebon