pretty: provide a strict ISO 8601 date format

pretty: provide a strict ISO 8601 date format

Git's "ISO" date format does not really conform to the ISO 8601
standard due to small differences, and it cannot be parsed by ISO
8601-only parsers, e.g. those of XML toolchains.

The output from "--date=iso" deviates from ISO 8601 in these ways:

  - a space instead of the `T` date/time delimiter
  - a space between time and time zone
  - no colon between hours and minutes of the time zone

Add a strict ISO 8601 date format for displaying committer and
author dates.  Use the '%aI' and '%cI' format specifiers and add
'--date=iso-strict' or '--date=iso8601-strict' date format names.

See http://thread.gmane.org/gmane.comp.version-control.git/255879 and
http://thread.gmane.org/gmane.comp.version-control.git/52414/focus=52585
for discussion.

Signed-off-by: Beat Bolli <bbolli@ewanet.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>