commit-graph.c: peel refs in 'add_ref_to_set'

While iterating references (to discover the set of commits to write to the commit-graph with 'git commit-graph write --reachable'), 'add_ref_to_set' can save 'fill_oids_from_commits()' some time by peeling the references beforehand. Move peeling out of 'fill_oids_from_commits()' and into 'add_ref_to_set()' to use 'peel_ref()' instead of 'deref_tag()'. Doing so allows the commit-graph machinery to use the peeled value from '$GIT_DIR/packed-refs' instead of having to load and parse tags. While we're at it, discard non-commit objects reachable from ref tips. This would be done automatically by 'fill_oids_from_commits()', but such functionality will be removed in a subsequent patch after the call to 'lookup_commit_reference_gently' is dropped (at which point a non-commit object in the commits oidset will become an error). Suggested-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>