Store empty (default) branch in branch cache, too.
Operations on this branch don't work otherwise.
Reading branches.cache had to be adjusted to allow an empty label.
Adjusted reading of the cache tip for symmetry, no functional change here.
#!/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