Add missing import "parse_date_or_fatal" to cmd/split-cmd.py

[xx4h]

If using bup split -n NAME -d STAMP FILE we currently get an error:

Traceback (most recent call last):
  File "/usr/lib/bup/cmd/bup-split", line 67, in <module>
    date = parse_date_or_fatal(opt.date, o.fatal)
NameError: name 'parse_date_or_fatal' is not defined

this patch does add the missing parse_date_or_fatal to from bup.helpers import

[xx4h]

Anything missing here?

[rlbdv]

Thanks for the help. In accordance with HACKING I've redirected this pull request to the list for discussion: https://groups.google.com/d/msg/bup-list/hwHfXQz4Fwg/A3kS5Nw-BAAJ

[rlbdv]

Pushed to master. Thanks.