Not sure why, but apparently SHA works better

It's very weird that source_commit.raw_commit and
rugged.branches[merge_request.target_branch].target
should be completely the same. I checked with ==
and other values which proved that both should be
the same, but still tests cannot pass for:
spec/services/merge_requests/refresh_service_spec.rb

I decided to give it up. We could just use SHA and
that works fine anyway.

app/models/repository.rb

@@ -903,8 +903,8 @@ class Repository
   def merge(user, merge_request, options = {})
     GitOperationService.new(user, self).with_branch(
       merge_request.target_branch) do |source_commit|
-      our_commit = source_commit.raw_commit
-      their_commit = rugged.lookup(merge_request.diff_head_sha)
+      our_commit = source_commit.sha
+      their_commit = merge_request.diff_head_sha
 
       raise 'Invalid merge target' unless our_commit
       raise 'Invalid merge source' unless their_commit