Use Repository#exists? in Repository#commit?

Just checking raw_repository is no longer accurate to determine if a
repository exists or not.

app/models/repository.rb

@@ -68,7 +68,7 @@ class Repository
   end
 
   def commit(id = 'HEAD')
-    return nil unless raw_repository
+    return nil unless exists?
     commit = Gitlab::Git::Commit.find(raw_repository, id)
     commit = Commit.new(commit, @project) if commit
     commit