Merge branch 'dw/check-ignore-sans-index'

"git check-ignore" follows the same rule as "git add" and "git
status" in that the ignore/exclude mechanism does not take effect
on paths that are already tracked.  With "--no-index" option, it
can be used to diagnose which paths that should have been ignored
have been mistakenly added to the index.

* dw/check-ignore-sans-index:
check-ignore: Add option to ignore index contents

diff --cc builtin/check-ignore.c
Simple merge

diff --cc t/t0008-ignores.sh
Simple merge