Merge branch 'bw/short-ref-strict'

Merge branch 'bw/short-ref-strict'

* bw/short-ref-strict:
  remote.c: use shorten_unambiguous_ref
  rev-parse: --abbrev-ref option to shorten ref name
  for-each-ref: utilize core.warnAmbiguousRefs for :short-format
  shorten_unambiguous_ref(): add strict mode