Merge branch 'jn/clone-add-empty-config-from-command-line' into maint

"git clone --config var=val" is a way to populate the
per-repository configuration file of the new repository, but it did
not work well when val is an empty string.  This has been fixed.

* jn/clone-add-empty-config-from-command-line:
  clone: handle empty config values in -c

builtin/clone.c

@@ -773,7 +773,9 @@ static int checkout(int submodule_progress)
 
 static int write_one_config(const char *key, const char *value, void *data)
 {
-	return git_config_set_multivar_gently(key, value ? value : "true", "^$", 0);
+	return git_config_set_multivar_gently(key,
+					      value ? value : "true",
+					      CONFIG_REGEX_NONE, 0);
 }
 
 static void write_config(struct string_list *config)