wireproto: don't expose changegroupsubset capability if not available
We just marked the changegroupsubset command as only available to
version 1 of the wire transports. There is a capability of the same
name of the command that indicates if the command is supported.
This commit teaches the capabilities code to conditionally emit that
capability depending on whether the command is available for the
current transport.
Most test output is reordering of capabilities. But the limited tests
for version 2 of the SSH protocol do show the capability disappearing.
Differential Revision: https://phab.mercurial-scm.org/D2486