tests/narrow-library.sh

perf: handle NameError for `pycompat.foo` when pycompat wasn't imported

On old Mercurial versions, we won't have a pycompat variable defined,
and then `pycompat.foo` will raise a NameError.

Differential Revision: https://phab.mercurial-scm.org/D6743

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$RUNTESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF