Display target branch on MR list when it is different from project's default

Fixes #3072

The screenshot below shows both previous and current UI state:

![branch-mr-both](/uploads/5c1f0519e42bae4655f3eee37edfc356/branch-mr-both.png)

First merge request has default branch as a target, second one has non default branch as target.

See merge request !1741