Merge branch 'dl/pretty-reference'

Merge branch 'dl/pretty-reference'

"git log" family learned "--pretty=reference" that gives the name
of a commit in the format that is often used to refer to it in log
messages.

* dl/pretty-reference:
  SubmittingPatches: use `--pretty=reference`
  pretty: implement 'reference' format
  pretty: add struct cmt_fmt_map::default_date_mode_type
  pretty: provide short date format
  t4205: cover `git log --reflog -z` blindspot
  pretty.c: inline initalize format_context
  revision: make get_revision_mark() return const pointer
  completion: complete `tformat:` pretty format
  SubmittingPatches: remove dq from commit reference
  pretty-formats.txt: use generic terms for hash
  SubmittingPatches: use generic terms for hash