parse_date_format(): convert a format name to an enum date_mode

Factor out the code to parse --date=<format> parameter to revision
walkers into a separate function, parse_date_format().  This function
is passed a string and converts it to an enum date_format:

- "relative"         => DATE_RELATIVE
- "iso8601" or "iso" => DATE_ISO8601
- "rfc2822"          => DATE_RFC2822
- "short"            => DATE_SHORT
- "local"            => DATE_LOCAL
- "default"          => DATE_NORMAL

In the event that none of these strings is found, the function die()s.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>