check-ignore: Add option to ignore index contents

check-ignore: Add option to ignore index contents

check-ignore currently shows how .gitignore rules would treat untracked
paths. Tracked paths do not generate useful output.  This prevents
debugging of why a path became tracked unexpectedly unless that path is
first removed from the index with `git rm --cached <path>`.

The option --no-index tells the command to bypass the check for the
path being in the index and hence allows tracked paths to be checked
too.

Whilst this behaviour deviates from the characteristics of `git add` and
`git status` its use case is unlikely to cause any user confusion.

Test scripts are augmented to check this option against the standard
ignores to ensure correct behaviour.

Signed-off-by: Dave Williams <dave@opensourcesolutions.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>