Jeff King [off-list ref] writes:
But what if we instead think of it not as "protocol v2" but as "can I
give the server some hints that it may end up ignoring", then we end up
with something more like:
C: please run upload-pack (btw, I'm only interested in refs/heads/foo)
S: advertisement + caps (hopefully limited to foo, but client is prepared to receive all)
... etc, as before ...
Nice. The caps that come back can tell us between the cases where
they only had refs/heads/foo and nothing else, or if they limited
their output to it among many others we told them to ignore, so
there is no ambiguity.
Or alternatively, I guess make this optional to start with, and
let early adopters turn it on and complain to their server vendors
for a while before flipping the default to on.
That sounds like a safe transition plan.