commit-graph.c: no lazy fetch in lookup_commit_in_graph()

commit-graph.c: no lazy fetch in lookup_commit_in_graph()

The commit-graph is used to opportunistically optimize accesses to
certain pieces of information on commit objects, and
lookup_commit_in_graph() tries to say "no" when the requested commit
does not locally exist by returning NULL, in which case the caller
can ask for (which may result in on-demand fetching from a promisor
remote) and parse the commit object itself.

However, it uses a wrong helper, repo_has_object_file(), to do so.
This helper not only checks if an object is mmediately available in
the local object store, but also tries to fetch from a promisor remote.
But the fetch machinery calls lookup_commit_in_graph(), thus causing an
infinite loop.

We should make lookup_commit_in_graph() expect that a commit given to it
can be legitimately missing from the local object store, by using the
has_object_file() helper instead.

Signed-off-by: Han Xin <hanxin.hx@bytedance.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>