Reset possible helper before reusing remote structure

If one had multiple URLs configured for remote with previous one
having forced helper but the subsequent one not, like:

url = foo::bar://baz
url = ssh://example/example.git

Then the subsequent URL is passed to foo helper, which isn't
correct. Fix it to be parsed normally by resetting foreign VCS
name before parsing the URL protocol.

Signed-off-by: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Signed-off-by: Junio C Hamano <gitster@pobox.com>