Merge branch 'ar/unconfuse-three-dots'

Merge branch 'ar/unconfuse-three-dots'

Ancient part of codebase still shows dots after an abbreviated
object name just to show that it is not a full object name, but
these ellipses are confusing to people who newly discovered Git
who are used to seeing abbreviated object names and find them
confusing with the range syntax.

* ar/unconfuse-three-dots:
  t2020: test variations that matter
  t4013: test new output from diff --abbrev --raw
  diff: diff_aligned_abbrev: remove ellipsis after abbreviated SHA-1 value
  t4013: prepare for upcoming "diff --raw --abbrev" output format change
  checkout: describe_detached_head: remove ellipsis after committish
  print_sha1_ellipsis: introduce helper
  Documentation: user-manual: limit usage of ellipsis
  Documentation: revisions: fix typo: "three dot" ---> "three-dot" (in line with "two-dot").