Remove invalid parents from the Commit object

What does this MR do and why?

Contributes to #478834 (closed)

Problem

It is possible that commits might have pointers to non-existing parents. An attempt to display such commit will lead to NoMethodError.

Solution

Filter out invalid commit objects from parents list.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

You can patch the code to emulate this problem.

1. Open app/models/commit.rb
2. Replace parent_ids array with an list of invalid commit shas. Like

   def parents
     # Usage of `reject` is intentional. `compact` doesn't work here, because of BatchLoader specifics
-    @parents ||= parent_ids.map { |oid| Commit.lazy(container, oid) }.reject(&:nil?)
+    @parents ||= ["111"].map { |oid| Commit.lazy(container, oid) }.reject(&:nil?)
   end

3. Visit any commit page (http://gdk.test:3000/gitlab-org/gitlab-test/-/commit/c7fbe50c7c7419d9701eebe64b1fdacc3df5b9dd)
4. It should load without any errors (parents list will be empty)