keepalive: Do not append _rbuf if _raw_readinto exists (issue6356)
The readline method append to the chunks the content of the _rbuf then there
is a loop that call _raw_read which on Python3 call readinto. But the readinto
version in mercurial append again the _rbuf content. So this creates the
duplicate content. This does not happen in Python2 because _raw_read does not
call readinto.
Differential Revision: https://phab.mercurial-scm.org/D8859