Merge branch 'sj/ref-fsck'

Merge branch 'sj/ref-fsck'

"git fsck" infrastructure has been taught to also check the sanity
of the ref database, in addition to the object database.

* sj/ref-fsck:
  fsck: add ref name check for files backend
  files-backend: add unified interface for refs scanning
  builtin/refs: add verify subcommand
  refs: set up ref consistency check infrastructure
  fsck: add refs report function
  fsck: add a unified interface for reporting fsck messages
  fsck: make "fsck_error" callback generic
  fsck: rename objects-related fsck error functions
  fsck: rename "skiplist" to "skip_oids"