stream-clone: check the version of streaming clone supported by the client
Make the server refuse to produce streaming clone bundle, if the client
doesn't specify the stream=v2 capability.
This is in preparation to introduce stream=v3.
As far as I can tell, this capability was added at the same time as
support for bundle2-based streaming pulls was added, so I don't expect
clients to break. (the modern client doesn't break, at any rate)
# Files that just need to be migrated to the formatter.
# Do not add new files here!
mercurial/cext/manifest.c
mercurial/cext/osutil.c
# Vendored code that we should never format:
syntax: glob
contrib/python-zstandard/**.c
contrib/python-zstandard/**.h
hgext/fsmonitor/pywatchman/**.c
mercurial/thirdparty/**.c
mercurial/thirdparty/**.h
mercurial/pythoncapi_compat.h