Refactor user info retrieval methods

fix warnings:
E_WARNING: Trying to access array offset on null/storage/lib/plugins/gitbacked/action/editcommit.php(96)

E_WARNING: Trying to access array offset on null/storage/lib/plugins/gitbacked/action/editcommit.php(100)

Author: ribsey

action/editcommit.php

@@ -117,14 +117,19 @@ private function commitFile($filePath, $message)
         }
     }
 
-    private function getAuthor()
-    {
-        return $GLOBALS['USERINFO']['name'];
+    private function getUserInfo($key) {
+        if (isset($GLOBALS['USERINFO'][$key])) {
+            return $GLOBALS['USERINFO'][$key];
+        }
+        return '';
     }
-
-    private function getAuthorMail()
-    {
-        return $GLOBALS['USERINFO']['mail'];
+    
+    private function getAuthor() {
+        return $this->getUserInfo('name');
+    }
+    
+    private function getAuthorMail() {
+        return $this->getUserInfo('mail');
     }
 
     private function computeLocalPath()