datefn: Don't overwrite files unless the new -f/--force option is used

5e76702e-13f8-11e3-819d-a088b4ddef28
datefn.t: Add tests for --force and overwriting

c3489a84-13fa-11e3-bbd0-a088b4ddef28
Merge branch 'datefn.add-force-option'

* datefn.add-force-option:
  datefn.t: Add tests for --force and overwriting
  datefn: Don't overwrite files unless the new -f/--force option is used

2d241ec4-13fb-11e3-bad8-a088b4ddef28