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>

remote.c

@@ -2761,10 +2761,18 @@ char *relative_url(const char *remote_url, const char *url,
 	int is_relative = 0;
 	int colonsep = 0;
 	char *out;
-	char *remoteurl = xstrdup(remote_url);
+	char *remoteurl;
 	struct strbuf sb = STRBUF_INIT;
-	size_t len = strlen(remoteurl);
+	size_t len;
 
+	if (!url_is_local_not_ssh(url) || is_absolute_path(url))
+		return xstrdup(url);
+
+	len = strlen(remote_url);
+	if (!len)
+		BUG("invalid empty remote_url");
+
+	remoteurl = xstrdup(remote_url);
 	if (is_dir_sep(remoteurl[len-1]))
 		remoteurl[len-1] = '\0';
 

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.
  */