ui: also swap sys.stdout with self.fout in _readline
In
17ffb30d9174, _readline was changed to output a space using
raw_input and this was done using sys.stdout directly, not self.fout.
This change broke the command server for JavaHg since it (and other
clients) would see a spurious ' ' on stdout and interpret this as an
unknown channel.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null