Convert commit_tree() to take strbuf as message

Convert commit_tree() to take strbuf as message

There wan't a way for commit_tree() to notice if the message the caller
prepared contained a NUL byte, as it did not take the length of the
message as a parameter. Use a pointer to a strbuf instead, so that we can
either choose to allow low-level plumbing commands to make commits that
contain NUL byte in its message, or forbid NUL everywhere by adding the
check in commit_tree(), in later patches.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>