notes: document length of fanout path with a constant

notes: document length of fanout path with a constant

We know that a fanned-out sha1 in a notes tree cannot be
more than "aa/bb/cc/...", and we have an assert() to confirm
that. But let's factor out that length into a constant so we
can be sure it is used consistently. And even though we
assert() earlier, let's replace a strcpy with xsnprintf, so
it is clear to a reader that all cases are covered.

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