Expire the exists cache before deletion to ensure project dir actually exists

Closes #14659

app/models/repository.rb

@@ -335,6 +335,8 @@ class Repository
 
   # Runs code just before a repository is deleted.
   def before_delete
+    expire_exists_cache
+
     expire_cache if exists?
 
     expire_root_ref_cache

spec/models/repository_spec.rb

@@ -558,7 +558,7 @@ describe Repository, models: true do
       end
 
       it 'flushes the exists cache' do
-        expect(repository).to receive(:expire_exists_cache)
+        expect(repository).to receive(:expire_exists_cache).twice
 
         repository.before_delete
       end