home_config_paths(): let the caller ignore xdg path

The caller can signal that it is not interested in learning
the location of $HOME/.gitconfig by passing global=NULL, but
there is no way to decline the path to the configuration
file based on $XDG_CONFIG_HOME.

Allow the caller to pass xdg=NULL to signal that it is not
interested in the XDG location.

Commit-message-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -148,10 +148,12 @@ void home_config_paths(char **global, char **xdg, char *file)
 			*global = mkpathdup("%s/.gitconfig", home);
 	}
 
-	if (!xdg_home)
-		*xdg = NULL;
-	else
-		*xdg = mkpathdup("%s/git/%s", xdg_home, file);
+	if (xdg) {
+		if (!xdg_home)
+			*xdg = NULL;
+		else
+			*xdg = mkpathdup("%s/git/%s", xdg_home, file);
+	}
 
 	free(to_free);
 }