Merge branch 'first-commit' into 'master'

MergeRequest#commits.last is actually first_commit

Using a method would be less confusing.

This was discovered in the process of figuring out https://gitlab.com/gitlab-org/gitlab-ee/issues/260

/cc @DouweM 

See merge request !453

app/models/merge_request.rb

@@ -611,7 +611,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def source_sha_parent
-    source_project.repository.commit(commits.last.sha).parents.first.sha
+    source_project.repository.commit(first_commit.sha).parents.first.sha
   end
 
   def ff_merge_possible?