Make for-each-ref allow atom names like "<name>:<something>"

Make for-each-ref allow atom names like "<name>:<something>"

In anticipation of supplying a per-field date format specifier, this
patch makes parse_atom() in builtin-for-each-ref.c allow atoms that have
a valid atom name (as determined by the valid_atom[] table) followed by
a colon, followed by an arbitrary string.

The arbitrary string is where the format for the atom will be specified.

Note, if different formats are specified for the same atom, multiple
entries will be made in the used_atoms table to allow them to be
distinguished by the grab_XXXX() functions.

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