remote: allow resetting url list

remote: allow resetting url list

Because remote.*.url is treated as a multi-valued key, there is no way
to override previous config. So for example if you have
remote.origin.url set to some wrong value, doing:

  git -c remote.origin.url=right fetch

would not work. It would append "right" to the list, which means we'd
still fetch from "wrong" (since subsequent values are used only as push
urls).

Let's provide a mechanism to reset the list, like we do for other
multi-valued keys (e.g., credential.helper, http.extraheaders, and
merge.suppressDest all use this "empty string means reset" pattern).

Reported-by: Mathew George <mathewegeorge@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>