Fix project page 404 after rename and removing new name from alias slugs

Summary:
When renaming a project to a slug already listed under Additional hashtags and explicitly also removing that to-become slug, accessing the project via the URL `/tag/projectname/` returned a 404 until someone added the current project name explicitly under "Additional hashtags" again.

In that case, do not remove the alternative hashtag to avoid the 404.

Closes T15636

Test Plan: See steps in https://we.phorge.it/T15636

Reviewers: O1 Blessed Committers, speck

Reviewed By: O1 Blessed Committers, speck

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15636

Differential Revision: https://we.phorge.it/D25453

src/applications/project/editor/PhabricatorProjectTransactionEditor.php

@@ -295,6 +295,12 @@ final class PhabricatorProjectTransactionEditor
   }
 
   public function removeSlugs(PhabricatorProject $project, array $slugs) {
+    // Do not allow removing the project's primary slug which the edit form
+    // may allow through a series of renames/moves. See T15636
+    if (($key = array_search($project->getPrimarySlug(), $slugs)) !== false) {
+      unset($slugs[$key]);
+    }
+
     if (!$slugs) {
       return;
     }