wt-status.c: rework the way changes to the index and work tree are summarized

wt-status.c: rework the way changes to the index and work tree are summarized

Introduce a new infrastructure to find and summarize changes in a single
string list, and rewrite wt_status_print_{updated,changed} functions using
it.

The goal of this change is to give more information on conflicted paths in
the status output.

Signed-off-by: Junio C Hamano <gitster@pobox.com>