wt-status: move many global settings to wt_status structure

wt-status: move many global settings to wt_status structure

Turn four global variables (wt_status_use_color, show_tracked_files,
wt_status_relative_paths, and wt_status_submodule_summary) into fields of
wt_status structure.  They can also lose "wt_status_" prefix.

Get rid of "untracked" field that was used only to keep track of otherwise
available information redundantly.

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