rhg: Fall back to Python on unsupported `rhg config <section>`
Printing an entire section (as opposed to a single value with
`rhg config foo.bar`) is not supported yet in Rust only.
Differential Revision: https://phab.mercurial-scm.org/D10098
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=$PYTHON "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF