cherry-pick: format help message as strbuf

cherry-pick: format help message as strbuf

This gets rid of the fixed-size buffer and an unchecked
sprintf. That sprintf is actually OK as the only
variable-sized thing put in it is an abbreviated sha1, which
is bounded at 40 characters. However, the next patch will
change that to something unbounded.

Note that this function now returns an allocated buffer
instead of a static one; however, it doesn't matter as the
only caller exits immediately.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>