keepalive: implement readinto()
This is part of the standard I/O interface. It is used by the framing
protocol. So we need to implement it so frames can be decoded.
Differential Revision: https://phab.mercurial-scm.org/D2984
To run the tests, do:
cd tests/
python run-tests.py
See https://mercurial-scm.org/wiki/WritingTests for
more information on writing tests.