Save the current HEAD into refs/heads/base if the stack is empty after
a patch has been deleted. This was not done before, which caused
refs/heads/base to not be updated after 'stg commit'. To guard against
existing repositories with no applied patches and HEAD !=
refs/heads/base, also do the update every time someone asks for the
name of refs/heads/base.
Signed-off-by: Karl Hasselström <kha@treskal.com>