log-tree: always use return value of strbuf_detach()

strbuf_detach() has been returning a pointer to a buffer even for empty
strbufs since 08ad56f3f0 ("strbuf: always return a non-NULL value from
strbuf_detach", 2012-10-18).  Use that feature in show_log() instead of
having it handle empty strbufs specially.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>