contrib/credential/netrc: work outside a repo

contrib/credential/netrc: work outside a repo

Currently, git-credential-netrc does not work outside of a git
repository. It fails with the following error:

	fatal: Not a git repository: . at /usr/share/perl5/Git.pm line 214.

There is no real reason why need to be within a repository, though.
Credential helpers should be able to work just fine outside the
repository as well.

Call the non-self version of config() so that git-credential-netrc no
longer needs to be run within a repository.

Signed-off-by: Denton Liu <redacted>
Signed-off-by: Junio C Hamano <redacted>