Remove repository graph log to fix slow cache updates after push event

Recalculating graph_log took minutes on a large repository since each
of the 6000 commits would need to calculate the diff. Since graph_log
does not appear to be used, remove it from the list of things to update.
For the Linux kernel repository, this reduced the ProjectCacheWorker time
from 8+ minutes to 16 seconds.

Closes #2016

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - Remove repository graph log to fix slow cache updates after push event (Stan Hu)
   - Fix label read access for unauthenticated users (Daniel Gerhardt)
   - Fix OAuth provider bug where GitLab would not go return to the redirect_uri after sign-in (Stan Hu)
   - Fix file upload dialog for comment editing (Daniel Gerhardt)

app/models/repository.rb

@@ -119,7 +119,7 @@ class Repository
   end
 
   def cache_keys
-    %i(size branch_names tag_names commit_count graph_log
+    %i(size branch_names tag_names commit_count
        readme version contribution_guide changelog license)
   end
 
@@ -144,24 +144,6 @@ class Repository
     end
   end
 
-  def graph_log
-    cache.fetch(:graph_log) do
-      commits = raw_repository.log(limit: 6000, skip_merges: true,
-                                   ref: root_ref)
-
-      commits.map do |rugged_commit|
-        commit = Gitlab::Git::Commit.new(rugged_commit)
-
-        {
-          author_name: commit.author_name,
-          author_email: commit.author_email,
-          additions: commit.stats.additions,
-          deletions: commit.stats.deletions,
-        }
-      end
-    end
-  end
-
   def lookup_cache
     @lookup_cache ||= {}
   end