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 % init
hg init
echo % commit
echo 'a' > a
hg ci -A -m test -u nobody -d '1 0'
echo % annotate -c
hg annotate -c a
echo % annotate -d
hg annotate -d a
echo % annotate -n
hg annotate -n a
echo % annotate -u
hg annotate -u a
echo % annotate -cdnu
hg annotate -cdnu a
cat <<EOF >>a
a
a
EOF
hg ci -ma1 -d '1 0'
hg cp a b
hg ci -mb -d '1 0'
cat <<EOF >> b
b
b
b
EOF
hg ci -mb2 -d '2 0'
echo % annotate b
hg annotate b
echo % annotate -nf b
hg annotate -nf b
hg up -C 2
cat <<EOF >> b
b
c
b
EOF
hg ci -mb2.1 -d '2 0'
hg merge
hg ci -mmergeb -d '3 0'
echo % annotate after merge
hg annotate -nf b
hg up -C 1
hg cp a b
cat <<EOF > b
a
z
a
EOF
hg ci -mc -d '3 0'
hg merge
cat <<EOF >> b
b
c
b
EOF
echo d >> b
hg ci -mmerge2 -d '4 0'
echo % annotate after rename merge
hg annotate -nf b
echo % linkrev vs rev
hg annotate -r tip a