Add git-repo-structure options to filter references

Similar to git-sizer(1), git-repo-structure should provide options to filter references.

--[no-]branches          process [don't process] branches
--[no-]tags              process [don't process] tags
--[no-]remotes           process [don't process] remote-tracking references
--[no-]notes             process [don't process] git-notes references
--[no-]stash             process [don't process] refs/stash