Make for-each-ref's grab_date() support per-atom formatting

Make for-each-ref's grab_date() support per-atom formatting

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>