Merge branch 'jk/credential-clear-config'

The credential.helper configuration variable is cumulative and
there is no good way to override it from the command line.  As
a special case, giving an empty string as its value now serves
as the signal to clear the values specified in various files.

* jk/credential-clear-config:
  credential: let empty credential specs reset helper list

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"))
-		string_list_append(&c->helpers, value);
-	else if (!strcmp(key, "username")) {
+	if (!strcmp(key, "helper")) {
+		if (*value)
+			string_list_append(&c->helpers, value);
+		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