Merge branch 'sg/completion-refs-speedup'

The refs completion for large number of refs has been sped up,
partly by giving up disambiguating ambiguous refs and partly by
eliminating most of the shell processing between 'git for-each-ref'
and 'ls-remote' and Bash's completion facility.

* sg/completion-refs-speedup:
  completion: speed up branch and tag completion
  completion: fill COMPREPLY directly when completing fetch refspecs
  completion: fill COMPREPLY directly when completing refs
  completion: let 'for-each-ref' sort remote branches for 'checkout' DWIMery
  completion: let 'for-each-ref' filter remote branches for 'checkout' DWIMery
  completion: let 'for-each-ref' strip the remote name from remote branches
  completion: let 'for-each-ref' and 'ls-remote' filter matching refs
  completion: don't disambiguate short refs
  completion: don't disambiguate tags and branches
  completion: support excluding full refs
  completion: support completing fully qualified non-fast-forward refspecs
  completion: support completing full refs after '--option=refs/<TAB>'
  completion: wrap __git_refs() for better option parsing
  completion: remove redundant __gitcomp_nl() options from _git_commit()