credential: let empty credential specs reset helper list

Sine the credential.helper key is a multi-valued config
list, there's no way to "unset" a helper once it's been set.
So if your system /etc/gitconfig sets one, you can never
avoid running it, but only add your own helpers on top.

Since an empty value for credential.helper is nonsensical
(it would just try to run "git-credential-"), we can assume
nobody is using it. Let's define it to reset the helper
list, letting you override lower-priority instances which
have come before.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1113,8 +1113,9 @@ commit.template::
 credential.helper::
 	Specify an external helper to be called when a username or
 	password credential is needed; the helper may consult external
-	storage to avoid prompting the user for the credentials. See
-	linkgit:gitcredentials[7] for details.
+	storage to avoid prompting the user for the credentials. Note
+	that multiple helpers may be defined. See linkgit:gitcredentials[7]
+	for details.
 
 credential.useHttpPath::
 	When acquiring credentials, consider the "path" component of an http

Documentation/gitcredentials.txt

@@ -106,6 +106,11 @@ variable, each helper will be tried in turn, and may provide a username,
 password, or nothing. Once Git has acquired both a username and a
 password, no more helpers will be tried.
 
+If `credential.helper` is configured to the empty string, this resets
+the helper list to empty (so you may override a helper set by a
+lower-priority config file by configuring the empty-string helper,
+followed by whatever set of helpers you would like).
+
 
 CREDENTIAL CONTEXTS
 -------------------

credential.c

@@ -63,9 +63,12 @@ static int credential_config_callback(const char *var, const char *value,
 		key = dot + 1;
 	}
 
-	if (!strcmp(key, "helper"))
+	if (!strcmp(key, "helper")) {
+		if (*value)
 			string_list_append(&c->helpers, value);
-	else if (!strcmp(key, "username")) {
+		else
+			string_list_clear(&c->helpers, 0);
+	} else if (!strcmp(key, "username")) {
 		if (!c->username)
 			c->username = xstrdup(value);
 	}

t/t0300-credentials.sh

@@ -298,4 +298,15 @@ test_expect_success 'helpers can abort the process' '
 	test_cmp expect stdout
 '
 
+test_expect_success 'empty helper spec resets helper list' '
+	test_config credential.helper "verbatim file file" &&
+	check fill "" "verbatim cmdline cmdline" <<-\EOF
+	--
+	username=cmdline
+	password=cmdline
+	--
+	verbatim: get
+	EOF
+'
+
 test_done