read info/{attributes,exclude} only when in repository

read info/{attributes,exclude} only when in repository

The low-level attribute and gitignore code will try to look
in $GIT_DIR/info for any repo-level configuration files,
even if we have not actually determined that we are in a
repository (e.g., running "git grep --no-index"). In such a
case they end up looking for ".git/info/attributes", etc.

This is generally harmless, as such a file is unlikely to
exist outside of a repository, but it's still conceptually
the wrong thing to do.

Let's detect this situation explicitly and skip reading the
file (i.e., the same behavior we'd get if we were in a
repository and the file did not exist).

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