ref-filter.h: add max_count and omit_empty to ref_format

ref-filter.h: add max_count and omit_empty to ref_format

Add an internal 'array_opts' struct to 'struct ref_format' containing
formatting options that pertain to the formatting of an entire ref array:
'max_count' and 'omit_empty'. These values are specified by the '--count'
and '--omit-empty' options, respectively, to 'for-each-ref'/'tag'/'branch'.
Storing these values in the 'ref_format' will simplify the consolidation of
ref array formatting logic across builtins in later patches.

Signed-off-by: Victoria Dye <vdye@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>