Create cross-reference for closing references on commits pushed to non-default branches.

I also revamped the tests on "closing reference commits" (= "Fixes #xxxx" for example).
Now, there are two different contexts:
- when the commits with "closing reference" are pushed to the default branch,
- when the commits with "closing reference" are pushed to a non-default branch.

Closes gitlab-org/gitlab-ce#2190.