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

mercurial/keepalive.py

@@ -542,4 +542,6 @@
             return line
 
         # No newline in local buffer. Read until we find one.
+        # readinto read via readinto will already return _rbuf
+        if self._raw_readinto is None:
             chunks = [self._rbuf]
@@ -545,4 +547,6 @@
             chunks = [self._rbuf]
+        else:
+            chunks = []
         i = -1
         readsize = self._rbufsize
         while True: