Merge branch 'ms/refs-optimize'

"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