gitignore: report access errors of exclude files

gitignore: report access errors of exclude files

When we try to access gitignore files, we check for their
existence with a call to "access". We silently ignore
missing files. However, if a file is not readable, this may
be a configuration error; let's warn the user.

For $GIT_DIR/info/excludes or core.excludesfile, we can just
use access_or_warn. However, for per-directory files we
actually try to open them, so we must add a custom warning.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>