Move mergablility check to Gitaly

Closes https://gitlab.com/gitlab-org/gitaly/issues/889

app/models/repository.rb

@@ -847,7 +847,7 @@ class Repository
     @root_ref_sha ||= commit(root_ref).sha
   end
 
-  delegate :merged_branch_names, :can_be_merged?, to: :raw_repository
+  delegate :merged_branch_names, to: :raw_repository
 
   def merge_base(first_commit_id, second_commit_id)
     first_commit_id = commit(first_commit_id).try(:id) || first_commit_id

lib/gitlab/git/repository.rb

@@ -1413,13 +1413,8 @@ module Gitlab
       end
 
       def can_be_merged?(source_sha, target_branch)
-        gitaly_migrate(:can_be_merged) do |is_enabled|
-          if is_enabled
-            gitaly_can_be_merged?(source_sha, find_branch(target_branch, true).target)
-          else
-            rugged_can_be_merged?(source_sha, target_branch)
-          end
-        end
+        target_sha = find_branch(target_branch, true).target
+        !gitaly_conflicts_client(source_sha, target_sha).conflicts?
       end
 
       def search_files_by_name(query, ref)
@@ -2236,14 +2231,6 @@ module Gitlab
         run_git(['fetch', remote_name], env: env).last.zero?
       end
 
-      def gitaly_can_be_merged?(their_commit, our_commit)
-        !gitaly_conflicts_client(our_commit, their_commit).conflicts?
-      end
-
-      def rugged_can_be_merged?(their_commit, our_commit)
-        !rugged.merge_commits(our_commit, their_commit).conflicts?
-      end
-
       def gitlab_projects_error
         raise CommandError, @gitlab_projects.output
       end

spec/models/repository_spec.rb

@@ -434,7 +434,6 @@ describe Repository do
   end
 
   describe '#can_be_merged?' do
-    shared_examples 'can be merged' do
     context 'mergeable branches' do
       subject { repository.can_be_merged?('0b4bc9a49b562e85de7cc9e834518ea6828729b9', 'master') }
 
@@ -466,15 +465,6 @@ describe Repository do
     end
   end
 
-    context 'when Gitaly can_be_merged feature is enabled' do
-      it_behaves_like 'can be merged'
-    end
-
-    context 'when Gitaly can_be_merged feature is disabled', :disable_gitaly do
-      it_behaves_like 'can be merged'
-    end
-  end
-
   describe '#commit' do
     context 'when ref exists' do
       it 'returns commit object' do