submodule: fix relative url parsing for scp-style origin

submodule: fix relative url parsing for scp-style origin

The function resolve_relative_url was not prepared to deal with an
scp-style origin 'user@host:path' in the case where 'path' is only a
single component.  Fix this by extending the logic that strips one
path component from the $remoteurl.

Also add tests for both styles of URLs.

Noticed-by: Jeffrey Phillips Freeman <jeffrey.freeman@syncleus.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>