only offer the source upgrade if it is writeable

this check will prevent the source upgrade rendering if the .git folder
is not writeable

core/upgrade/index.php

@@ -142,7 +142,7 @@ echo "<br><br>";
 
 echo "<form name='frm' method='post' action=''>\n";
 
-if (permission_exists("upgrade_source") && !is_dir("/usr/share/examples/fusionpbx")) {
+if (permission_exists("upgrade_source") && !is_dir("/usr/share/examples/fusionpbx") && is_writeable($_SERVER["PROJECT_ROOT"]."/.git")) {
 	echo "<table width='100%' border='0' cellpadding='0' cellspacing='0'>\n";
 	echo "<tr>\n";
 	echo "	<td width='30%' class='vncell'>\n";