grab_date() gets an extra parameter - atomname; this extra parameter is
checked to see if it has a ":<format>" extra component in it, and if so
that "<format>" string is passed to parse_date_format() to produce an
enum date_mode value which is then further passed to show_date().
In short it allows the user of git-for-each-ref to do things like this:
$ git-for-each-ref --format='%(taggerdate:default)' refs/tags/v1.5.2
Sun May 20 00:30:42 2007 -0700
$ git-for-each-ref --format='%(taggerdate:relative)' refs/tags/v1.5.2
4 months ago
$ git-for-each-ref --format='%(taggerdate:short)' refs/tags/v1.5.2
2007-05-20
$ git-for-each-ref --format='%(taggerdate:local)' refs/tags/v1.5.2
Sun May 20 08:30:42 2007
$ git-for-each-ref --format='%(taggerdate:iso8601)' refs/tags/v1.5.2
2007-05-20 00:30:42 -0700
$ git-for-each-ref --format='%(taggerdate:rfc2822)' refs/tags/v1.5.2
Sun, 20 May 2007 00:30:42 -0700
The default, when no ":<format>" is specified is ":default", leaving the
existing behaviour unchanged.
Signed-off-by: Andy Parkins <andyparkins@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>