revision: use commit graph in get_reference()

revision: use commit graph in get_reference()

When fetching into a repository, a connectivity check is first made by
check_exist_and_connected() in builtin/fetch.c that runs:

  git rev-list --objects --stdin --not --all --quiet <(list of objects)

If the client repository has many refs, this command can be slow,
regardless of the nature of the server repository or what is being
fetched. A profiler reveals that most of the time is spent in
setup_revisions() (approx. 60/63), and of the time spent in
setup_revisions(), most of it is spent in parse_object() (approx.
49/60). This is because setup_revisions() parses the target of every ref
(from "--all"), and parse_object() reads the buffer of the object.

Reading the buffer is unnecessary if the repository has a commit graph
and if the ref points to a commit (which is typically the case). This
patch uses the commit graph wherever possible; on my computer, when I
run the above command with a list of 1 object on a many-ref repository,
I get a speedup from 1.8s to 1.0s.

Another way to accomplish this effect would be to modify parse_object()
to use the commit graph if possible; however, I did not want to change
parse_object()'s current behavior of always checking the object
signature of the returned object.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>