checkout,merge: disallow overwriting ignored files with --no-overwrite-ignore

checkout,merge: disallow overwriting ignored files with --no-overwrite-ignore

Ignored files usually are generated files (e.g. .o files) and can be
safely discarded. However sometimes users may have important files in
working directory, but still want a clean "git status", so they mark
them as ignored files. But in this case, these files should not be
overwritten without asking first.

Enable this use case with --no-overwrite-ignore, where git only sees
tracked and untracked files, no ignored files. Those who mix
discardable ignored files with important ones may have to sort it out
themselves.

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