wt-status.c: rename rename-related fields in wt_status_change_data

wt-status.c: rename rename-related fields in wt_status_change_data

These field "head_path" is used for rename display only. In the next
patch we introduce another rename pair where the rename source is no
longer HEAD. Rename it to something more generic.

While at there, rename "score" as well and store the rename diff code
in a separate field instead of hardcoding key[0] (i.e. diff-index) in
porcelain v2 code.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>