remote: allow relative_url() to return an absolute url
When the 'url' parameter was absolute, the previous implementation would
concatenate 'remote_url' with 'url'. Instead, we want to return 'url' in
this case.
The documentation now discusses what happens when supplying two
absolute URLs.
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Derrick Stolee <derrickstolee@github.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>