It is the expectation that credential helpers be liberal in what they
accept and conservative in what they return, to allow for future growth
and evolution of the protocol/interaction.

All of the other helpers (store, cache, osxkeychain, libsecret,
gnome-keyring) except `netrc` currently ignore any credential lines
that are not recognised, whereas the Windows helper (wincred) instead
dies.

Fix the discrepancy and ignore unknown lines in the wincred helper.

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