Merge branch 'ps/reftable-exclude'

Merge branch 'ps/reftable-exclude'

The reftable backend learned to more efficiently handle exclude
patterns while enumerating the refs.

* ps/reftable-exclude:
  refs/reftable: wire up support for exclude patterns
  reftable/reader: make table iterator reseekable
  t/unit-tests: introduce reftable library
  Makefile: stop listing test library objects twice
  builtin/receive-pack: fix exclude patterns when announcing refs
  refs: properly apply exclude patterns to namespaced refs