Merge branch 'hv/ref-filter-misc'

Merge branch 'hv/ref-filter-misc'

The "--format=" option to the "for-each-ref" command and friends
learned a few more tricks, e.g. the ":short" suffix that applies to
"objectname" now also can be used for "parent", "tree", etc.

* hv/ref-filter-misc:
  ref-filter: add `sanitize` option for 'subject' atom
  pretty: refactor `format_sanitized_subject()`
  ref-filter: add `short` modifier to 'parent' atom
  ref-filter: add `short` modifier to 'tree' atom
  ref-filter: rename `objectname` related functions and fields
  ref-filter: modify error messages in `grab_objectname()`
  ref-filter: refactor `grab_objectname()`
  ref-filter: support different email formats