t5616: cover case of client having delta base

t5616: cover case of client having delta base

When fetching into a partial clone, Git first prefetches missing
REF_DELTA bases from the promisor remote. (This feature was introduced
in [1].) But as can be seen in a recent test coverage report [2], the
case in which a REF_DELTA base is already present is not covered by
tests.

Extend the tests slightly to cover this case.

[1] 8a30a1efd1 ("index-pack: prefetch missing REF_DELTA bases",
2019-05-15).
[2] https://public-inbox.org/git/396091fc-5572-19a5-4f18-61c258590dd5@gmail.com/

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