Change default prefix to single space

Since the message part gets trimmed its fine to use the single space
default. In case it's at the beginning of the line it will be trimmed
away.

Author: sebastianfeldmann

src/Hook/Message/Action/InjectIssueKeyFromBranch.php

@@ -93,7 +93,7 @@ private function createNewCommitMessage(Options $options, CommitMessage $msg, st
         // let's figure out where to put the issueID
         $target = $options->get('into', 'body');
         $mode   = $options->get('mode', 'append');
-        $prefix = $options->get('prefix', '');
+        $prefix = $options->get('prefix', ' ');
 
         // overwrite either subject or body
         $newMsgData          = ['subject' => $msg->getSubject(), 'body' => $msg->getBody()];