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-tree.c

@@ -56,7 +56,7 @@ int cmd_commit_tree(int argc, const char **argv, const char *prefix)
 	if (strbuf_read(&buffer, 0, 0) < 0)
 		die_errno("git commit-tree: failed to read");
 
-	if (commit_tree(buffer.buf, tree_sha1, parents, commit_sha1, NULL)) {
+	if (commit_tree(&buffer, tree_sha1, parents, commit_sha1, NULL)) {
 		strbuf_release(&buffer);
 		return 1;
 	}