Allow setting branch refs for MR

app/models/merge_request.rb

@@ -238,12 +238,12 @@ class MergeRequest < ActiveRecord::Base
 
   def source_branch_head
     source_branch_ref = @source_branch_sha || source_branch
-    source_project.repository.commit(source_branch) if source_branch_ref
+    source_project.repository.commit(source_branch_ref) if source_branch_ref
   end
 
   def target_branch_head
     target_branch_ref = @target_branch_sha || target_branch
-    target_project.repository.commit(target_branch) if target_branch_ref
+    target_project.repository.commit(target_branch_ref) if target_branch_ref
   end
 
   def target_branch_sha