commit: correctly respect skip-worktree bit

commit: correctly respect skip-worktree bit

Commit b4d1690 (Teach Git to respect skip-worktree bit (reading part))
fails to make "git commit -- a b c" respect skip-worktree
(i.e. not committing paths that are skip-worktree). This is because
when the index is reset back to HEAD, all skip-worktree information is
gone.

This patch saves skip-worktree information in the string list of
committed paths, then reuse it later on to skip skip-worktree paths.

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