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

hg
hg -q
hg help
hg -q help
hg add -h
hg add --skjdfks
hg help diff
hg help status
hg -q help status
hg help foo
hg skjdfks

exit 0