snapshot: also consider the snapshot chain of one unrelated revision
To maximize the chance of good delta chain reuse, we inject an unrelated delta
chain into our search. To do so, we search for the highest revision unrelated
to the parents of the current revision and use its snapshot chain too.
Adding this extra snapshot into the mix can have a performance impact. We'll
deal with performance impact in a later series.
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF