remote: allow resetting url list

Because remote.*.url is treated as a multi-valued key, there is no way
to override previous config. So for example if you have
remote.origin.url set to some wrong value, doing:

  git -c remote.origin.url=right fetch

would not work. It would append "right" to the list, which means we'd
still fetch from "wrong" (since subsequent values are used only as push
urls).

Let's provide a mechanism to reset the list, like we do for other
multi-valued keys (e.g., credential.helper, http.extraheaders, and
merge.suppressDest all use this "empty string means reset" pattern).

Reported-by: Mathew George <mathewegeorge@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/remote.txt

@@ -8,13 +8,16 @@ remote.<name>.url::
 	linkgit:git-push[1]. A configured remote can have multiple URLs;
 	in this case the first is used for fetching, and all are used
 	for pushing (assuming no `remote.<name>.pushurl` is defined).
+	Setting this key to the empty string clears the list of urls,
+	allowing you to override earlier config.
 
 remote.<name>.pushurl::
 	The push URL of a remote repository.  See linkgit:git-push[1].
 	If a `pushurl` option is present in a configured remote, it
 	is used for pushing instead of `remote.<name>.url`. A configured
 	remote can have multiple push URLs; in this case a push goes to
-	all of them.
+	all of them. Setting this key to the empty string clears the
+	list of urls, allowing you to override earlier config.
 
 remote.<name>.proxy::
 	For remotes that require curl (http, https and ftp), the URL to

remote.c

@@ -63,12 +63,18 @@ static char *alias_url(const char *url, struct rewrites *r)
 
 static void add_url(struct remote *remote, const char *url)
 {
+	if (*url)
 		strvec_push(&remote->url, url);
+	else
+		strvec_clear(&remote->url);
 }
 
 static void add_pushurl(struct remote *remote, const char *pushurl)
 {
+	if (*pushurl)
 		strvec_push(&remote->pushurl, pushurl);
+	else
+		strvec_clear(&remote->pushurl);
 }
 
 static void add_pushurl_alias(struct remote_state *remote_state,

t/t5505-remote.sh

@@ -1492,4 +1492,40 @@ test_expect_success 'refs/remotes/* <src> refspec and unqualified <dst> DWIM and
 	)
 '
 
+test_expect_success 'empty config clears remote.*.url list' '
+	test_when_finished "git config --remove-section remote.multi" &&
+	git config --add remote.multi.url wrong-one &&
+	git config --add remote.multi.url wrong-two &&
+	git -c remote.multi.url= \
+	    -c remote.multi.url=right-one \
+	    -c remote.multi.url=right-two \
+	    remote show -n multi >actual.raw &&
+	grep URL actual.raw >actual &&
+	cat >expect <<-\EOF &&
+	  Fetch URL: right-one
+	  Push  URL: right-one
+	  Push  URL: right-two
+	EOF
+	test_cmp expect actual
+'
+
+test_expect_success 'empty config clears remote.*.pushurl list' '
+	test_when_finished "git config --remove-section remote.multi" &&
+	git config --add remote.multi.url right &&
+	git config --add remote.multi.url will-be-ignored &&
+	git config --add remote.multi.pushurl wrong-push-one &&
+	git config --add remote.multi.pushurl wrong-push-two &&
+	git -c remote.multi.pushurl= \
+	    -c remote.multi.pushurl=right-push-one \
+	    -c remote.multi.pushurl=right-push-two \
+	    remote show -n multi >actual.raw &&
+	grep URL actual.raw >actual &&
+	cat >expect <<-\EOF &&
+	  Fetch URL: right
+	  Push  URL: right-push-one
+	  Push  URL: right-push-two
+	EOF
+	test_cmp expect actual
+'
+
 test_done