copies: refactor symmetricdifference as _findlimit
We only need to track the lowest revision seen, which makes things simpler.
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "churn=" >> $HGRCPATH
echo % create test repository
hg init repo
cd repo
echo a > a
hg ci -Am adda -u user1
echo b >> a
echo b > b
hg ci -Am addb -u user2
echo c >> a
echo c >> b
echo c > c
hg ci -Am addc -u user3
echo % churn all
hg churn
echo % churn up to rev 1
hg churn -r :1
echo % churn with aliases
cat > ../aliases <<EOF
user1 alias1
user3 alias3
EOF
hg churn --aliases ../aliases
echo % churn with column specifier
COLUMNS=40 hg churn