diff: skip GITLINK when lazy fetching missing objs

diff: skip GITLINK when lazy fetching missing objs

In 7fbbcb21b1 ("diff: batch fetching of missing blobs", 2019-04-08),
diff was taught to batch the fetching of missing objects when operating
on a partial clone, but was not taught to refrain from fetching
GITLINKs. Teach diff to check if an object is a GITLINK before including
it in the set to be fetched.

(As stated in the commit message of that commit, unpack-trees was also
taught a similar thing prior, but unpack-trees correctly checks for
GITLINK before including objects in the set to be fetched.)

Signed-off-by: Junio C Hamano <gitster@pobox.com>