fetch-object: unify fetch_object[s] functions

fetch-object: unify fetch_object[s] functions

There are fetch_object() and fetch_objects() helpers in
fetch-object.h; as the latter takes "struct oid_array",
the former cannot be made into a thin wrapper around the
latter without an extra allocation and set-up cost.

Update fetch_objects() to take an array of "struct object_id"
and number of elements in it as separate parameters, remove
fetch_object(), and adjust all existing callers of these
functions to use the new fetch_objects().

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