Merge branch 'vd/for-each-ref-sort-with-formatted-timestamp'

"git branch" and friends learned to use the formatted text as
sorting key, not the underlying timestamp value, when the --sort
option is used with author or committer timestamp with a format
specifier (e.g., "--sort=creatordate:format:%H:%M:%S").

* vd/for-each-ref-sort-with-formatted-timestamp:
  ref-filter.c: sort formatted dates by byte value