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>

builtin/commit.c

@@ -1483,7 +1483,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		exit(1);
 	}
 
-	if (commit_tree(sb.buf, active_cache_tree->sha1, parents, sha1,
+	if (commit_tree(&sb, active_cache_tree->sha1, parents, sha1,
 			author_ident.buf)) {
 		rollback_index_files();
 		die(_("failed to write commit object"));