Speed up suggested approvers

Instead of performing `git log` for each file that's changed, do it for
all files at once. This stops git having to load large packfiles once
for each file in an MR.

This is always a suggestion, so the change in behaviour should be
fine. Increase the number of commits counted by squaring it, to allow
for the fewer calls to `git log`.

changelogs/unreleased-ee/1811-very-poor-performance-creating-merge-request-regression-in-9-0-often-hits-60-seconds-timeout-on-small-requests.yml

+---
+title: Speed up suggested approvers on MR creation
+merge_request:
+author:

lib/gitlab/authority_analyzer.rb

 module Gitlab
   class AuthorityAnalyzer
-    COMMITS_TO_CONSIDER = 5
+    COMMITS_TO_CONSIDER = 25
 
     def initialize(merge_request)
       @merge_request = merge_request
@@ -19,11 +19,9 @@ module Gitlab
     def involved_users
       @repo = @merge_request.target_project.repository
 
-      list_of_involved_files.each do |path|
-        @repo.commits(@merge_request.target_branch, path: path, limit: COMMITS_TO_CONSIDER).each do |commit|
-          if commit.author && commit.author != @merge_request.author
-            @users[commit.author] += 1
-          end
+      @repo.commits(@merge_request.target_branch, path: list_of_involved_files, limit: COMMITS_TO_CONSIDER).each do |commit|
+        if commit.author && commit.author != @merge_request.author
+          @users[commit.author] += 1
         end
       end
     end