Fix fatal error with long tags

See issue #696

src/Util/Routing.php

@@ -39,10 +39,12 @@ class Routing
              $slashPosition === 40)) {
             // We may have a commit hash as our commitish.
             $hash = substr($commitishPath, 0, 40);
+            if (preg_match('/[^a-zA-Z0-9]/i', $hash) === 0) {
                 if ($repository->hasCommit($hash)) {
                     $commitish = $hash;
                 }
             }
+        }
 
         if ($commitish === null) {
             $branches = $repository->getBranches();