Merge branch 'cc/rev-list-allow-missing-tips'

Merge branch 'cc/rev-list-allow-missing-tips'

"git rev-list --missing=print" has learned to optionally take
"--allow-missing-tips", which allows the objects at the starting
points to be missing.

* cc/rev-list-allow-missing-tips:
  revision: fix --missing=[print|allow*] for annotated tags
  rev-list: allow missing tips with --missing=[print|allow*]
  t6022: fix 'test' style and 'even though' typo
  oidset: refactor oidset_insert_from_set()
  revision: clarify a 'return NULL' in get_reference()