netrc: ignore unknown lines (do not die)

netrc: ignore unknown lines (do not die)

Contrary to the documentation on credential helpers, as well as the help
text for git-credential-netrc itself, this helper will `die` when
presented with an unknown property/attribute/token.

Correct the behaviour here by skipping and ignoring any tokens that are
unknown. This means all helpers in the tree are consistent and ignore
any unknown credential properties/attributes.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>