Merge branch 'jt/partial-clone-missing-ref-delta-base' into maint

"git fetch" into a lazy clone forgot to fetch base objects that are
necessary to complete delta in a thin packfile, which has been
corrected.

* jt/partial-clone-missing-ref-delta-base:
  t5616: cover case of client having delta base
  t5616: use correct flag to check object is missing
  index-pack: prefetch missing REF_DELTA bases
  t5616: refactor packfile replacement