HEAD is not explicitly used as a starting commit for
calculating reachability, so if it's detached and reflogs
are disabled it may be pruned.

Add tests which demonstrate it. Test 'prune: prune former HEAD after checking
out branch' also reverts changes to repository.
Signed-off-by: NMax Kirillov <max@max630.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>