Merge branch 'ms/refs-optimize' into next

Merge branch 'ms/refs-optimize' into next

"git refs optimize" is added for not very well explained reason
despite it does the same thing as "git pack-refs"...

* ms/refs-optimize:
  t: add test for git refs optimize subcommand
  t0601: refactor tests to be shareable
  builtin/refs: add optimize subcommand
  doc: pack-refs: factor out common options
  builtin/pack-refs: factor out core logic into a shared library
  builtin/pack-refs: convert to use the generic refs_optimize() API
  reftable-backend: implement 'optimize' action
  files-backend: implement 'optimize' action
  refs: add a generic 'optimize' API