Pick up direct links to issues/MRs as references.

lib/gitlab/markdown/abstract_reference_filter.rb

...lib/gitlab/markdown/commit_range_reference_filter_spec.rb

spec/lib/gitlab/markdown/commit_reference_filter_spec.rb

spec/lib/gitlab/markdown/issue_reference_filter_spec.rb

spec/lib/gitlab/markdown/label_reference_filter_spec.rb

...ib/gitlab/markdown/merge_request_reference_filter_spec.rb

spec/lib/gitlab/markdown/snippet_reference_filter_spec.rb

spec/lib/gitlab/markdown/user_reference_filter_spec.rb