commit-graph.c: simplify 'fill_oids_from_commits'
In the previous handful of commits, both 'git commit-graph write
--reachable' and '--stdin-commits' learned to peel tags down to the
commits which they refer to before passing them into the commit-graph
internals.
This makes the call to 'lookup_commit_reference_gently()' inside of
'fill_oids_from_commits()' a noop, since all OIDs are commits by that
point.
As such, remove the call entirely, as well as the progress meter, which
has been split and moved out to the callers in the aforementioned
earlier commits.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>