count-objects: report alternates via verbose mode

count-objects: report alternates via verbose mode

There's no way to get the list of alternates that git
computes internally; our tests only infer it based on which
objects are available. In addition to testing, knowing this
list may be helpful for somebody debugging their alternates
setup.

Let's add it to the "count-objects -v" output. We could give
it a separate flag, but there's not really any need.
"count-objects -v" is already a debugging catch-all for the
object database, its output is easily extensible to new data
items, and printing the alternates is not expensive (we
already had to find them to count the objects).

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