merge: fix spurious merges for copies in linear updates
We make better use of contexts to accurately identify copies that
don't need merges.
Add a simple test and update other tests.
#!/bin/sh
echo "[extensions]" >> $HGRCPATH
echo "mq=" >> $HGRCPATH
cat > $HGTMP/false <<EOF
#!/bin/sh
exit 1
EOF
chmod +x $HGTMP/false
hg init foo
cd foo
echo foo > foo
hg add foo
# mq may keep a reference to the repository so __del__ will not be called
# and .hg/journal.dirstate will not be deleted:
HGEDITOR=$HGTMP/false hg ci
HGEDITOR=$HGTMP/false hg ci
exit 0