better Head detection if refs/heads is pointing to non-existing master

lib/Git/Repository.php

@@ -422,14 +422,21 @@ class Repository
         else  {
           return 'master';
         }
+        // Find first existing branch
         foreach (explode("\n", $file) as $line) {
             $m = array();
             if (preg_match('#ref:\srefs/heads/(.+)#', $line, $m)) {
-                return $m[1];
+                if ($this->hasBranch($m[1])) {
+                  return $m[1];
+                }
             }
         }
 
         // Default to something sane if in a detached HEAD state.
+        $branches = $this->getBranches();
+        if (!empty($branches)) {
+          return current($branches);
+        }
         return 'master';
     }