for-each-ref: rename 'refinfo' to 'ref_array_item'

for-each-ref: rename 'refinfo' to 'ref_array_item'

Rename 'refinfo' to 'ref_array_item' as a preparatory step for
introduction of new structures in the forthcoming patch.

Re-order the fields in 'ref_array_item' so that refname can be
eventually converted to a FLEX_ARRAY.

Mentored-by: Christian Couder <redacted>
Mentored-by: Matthieu Moy <redacted>
Signed-off-by: Karthik Nayak <redacted>
Reviewed-by: Matthieu Moy <redacted>
Signed-off-by: Junio C Hamano <redacted>