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>
diff --git a/remote.h b/remote.h
index f18fd27..dd44024 100644
--- a/remote.h
+++ b/remote.h
@@ -434,6 +434,7 @@ void apply_push_cas(struct push_cas_option *, struct remote *, struct ref *);
  * http://a.com/b  ../../../c       http:/c          error out
  * http://a.com/b  ../../../../c    http:c           error out
  * http://a.com/b  ../../../../../c    .:c           error out
+ * http://a.com/b  http://d.org/e   http://d.org/e   as is
  * NEEDSWORK: Given how chop_last_dir() works, this function is broken
  * when a local part has a colon in its path component, too.
  */