tests/narrow-library.sh

tests: show poor error message for  `hg cp -A --at-rev . non-existent dst`

Differential Revision: https://phab.mercurial-scm.org/D8495

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