checkout: don't rfc2047-encode oneline on detached HEAD

checkout: don't rfc2047-encode oneline on detached HEAD

When calling pretty_print_commit, there is an implicit
assumption that passing in a non-NULL "subject" variable
for oneline or email formats means that the output is part
of a subject and therefore "subject" to rfc2047 encoding.
This is not the desired effect when reporting the movement
of detached HEAD.

Signed-off-by: Junio C Hamano <gitster@pobox.com>