Introduce a controller concern for finding commit in Merge Request controllers

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The following discussion from !173382 (merged) should be addressed:

• @patrickbajao started a discussion: (+2 comments)

  I was thinking about this more today and I feel that it's better to have a controller concern something like WithMergeRequestCommit and we move the #commit method there. We can then include this concern in both MergeRequestsController and DiffsController. It would look something like:

  module WithMergeRequestCommit
    def commit
      commit_id = params[:commit_id].presence
      return unless commit_id
  
      return unless @merge_request.all_commits.exists?(sha: commit_id) ||
        @merge_request.recent_context_commits.map(&:id).include?(commit_id)
  
      @commit ||= @project.commit(commit_id)
    end
  end
  
  class DiffsController
    include WithMergeRequestCommit
  end
  
  class MergeRequestsController
    include WithMergeRequestCommit
  end

  Doing it this way means if a controller needs the #commit method they can include the concern. Compared to what we have now wherein all controllers inheriting from MergeRequests::ApplicationController have access to #commit even if they don't need it, I think it's better.