tests/narrow-library.sh

tags: behave better if a tags cache entry is partially written

This is done by discarding any partial cache entry, instead of
filling the partial cache entry with 0xff before.

Differential Revision: https://phab.mercurial-scm.org/D8095

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