log-tree: the previous one is still not quite right

log-tree: the previous one is still not quite right

The correct output would have NUL after each commit, so "-z --format=%s"
would have a single-liner subject with the line-terminating LF replaced
with NUL, and "-p/--stat -z --format=%s" would have a single-liner subject
with its line-terminating LF, followed by the diff/diffstat in which the
terminating LF of the last line is replaced with NUL, but to be consistent
with what "-p/--stat -z --pretty=format:%s" does, I think it is OK to
append NUL to the diff/diffstat part instead of replacing its last LF with
NUL.

The added test shows the update is still not right for "-p -z --format".

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