t1092: update a write-tree test

t1092: update a write-tree test

* 'on all' in the title of the test 'write-tree on all' was unclear;
remove it.

* Add a baseline 'test_all_match git write-tree' before making any
changes to the index, providing a reference point for the 'write-tree'
prior to any modifications.

* Add a comparison of the output of 'git status --porcelain=v2' to test
the working tree after 'write-tree' exits.

* Ensure SKIP_WORKTREE files weren't materialized on disk by using
"test_path_is_missing".

Signed-off-by: Shuqi Liang <cheskaqiqi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>