The name of the blob object that stores the filter specification
for sparse cloning/fetching was interpreted in a wrong place in the
code, causing Git to abort.

* jk/partial-clone-sparse-blob:
  list-objects-filter: use empty string instead of NULL for sparse "base"
  list-objects-filter: give a more specific error sparse parsing error
  list-objects-filter: delay parsing of sparse oid
  t5616: test cloning/fetching with sparse:oid=<oid> filter