bundle2: advertise bundle2 caps in server capabilities
We can now retrieve them from the server during push. The capabilities are
encoded the same way as in `replycaps` part (with an extra layer of urlquoting
to escape separators).
all()
draft()
::tip
draft() and ::tip
0::tip
roots(0::tip)
author(lmoscovicz)
author(mpm)
author(lmoscovicz) or author(mpm)
tip:0
max(tip:0)
min(0:tip)
0::
min(0::)
roots((tip~100::) - (tip~100::tip))
::p1(p1(tip))::