keepalive: be more careful about self._rbuf when calling super impls In Python 3, HTTPResponse implements read() in terms of readinto(), which was calling back into our readinto(), which duplicates self._rbuf if it's not empty. Before calling into super's read(), ensure self._rbuf is empty. Inheritance is bad, and undocumented self-use of your public API is one of many reasons. Differential Revision: https://phab.mercurial-scm.org/D4728