Improve performance of index action for Projects::CompareController

Problem

https://gitlab.com/gitlab-org/gitlab/-/compare is slow.

Description

The performance problem is caused by a DB query that loads avatars for all fetch projects on this page.

Screenshot_2022-05-31_at_17.57.42

We load all forked projects on compare page to show their names in the select box. But that also triggers avatar loading for each of them (because of after_initialize for Project model). For projects with many forks this query becomes a performance problem.

Proposal

Load avatars only for the current project and its namespace.