Refactor can_be_merged logic for merge request

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request.rb

@@ -209,13 +209,7 @@ class MergeRequest < ActiveRecord::Base
       if for_fork?
         Gitlab::Satellite::MergeAction.new(self.author, self).can_be_merged?
       else
-        rugged = project.repository.rugged
-        our_commit = rugged.branches[target_branch].target
-        their_commit = rugged.branches[source_branch].target
-
-        if our_commit && their_commit
-          !rugged.merge_commits(our_commit, their_commit).conflicts?
-        end
+        project.repository.can_be_merged?(source_branch, target_branch)
       end
 
     if can_be_merged

app/models/repository.rb

@@ -422,6 +422,15 @@ class Repository
     }
   end
 
+  def can_be_merged?(source_branch, target_branch)
+    our_commit = rugged.branches[target_branch].target
+    their_commit = rugged.branches[source_branch].target
+
+    if our_commit && their_commit
+      !rugged.merge_commits(our_commit, their_commit).conflicts?
+    end
+  end
+
   private
 
   def cache

spec/models/repository_spec.rb

@@ -33,4 +33,18 @@ describe Repository do
       it { is_expected.to be_nil }
     end
   end
+
+  describe :can_be_merged? do
+    context 'mergeable branches' do
+      subject { repository.can_be_merged?('feature', 'master') }
+
+      it { is_expected.to be_truthy }
+    end
+
+    context 'non-mergeable branches' do
+      subject { repository.can_be_merged?('feature_conflict', 'feature') }
+
+      it { is_expected.to be_falsey }
+    end
+  end
 end