Merge branch 'jc/name-rev-stdin'

Merge branch 'jc/name-rev-stdin'

Using "git name-rev --stdin" as an example, improve the framework to
prepare tests to pretend to be in the future where the breaking
changes have already happened.

* jc/name-rev-stdin:
  name-rev: remove "--stdin" support
  t6120: further modernize
  t6120: avoid hiding "git" exit status
  t: introduce WITH_BREAKING_CHANGES prerequisite
  t: extend test_lazy_prereq
  t: document test_lazy_prereq