tests/narrow-library.sh

tests: fix test-patch.t on pickier /bin/sh implementations

This is a graft of 0b39edeff033 and f44306940c94 from default because
I'm tired of seeing the FreeBSD build be red on stable. See those
revisions for details on what's going on here.

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