Fix layout issue in header title truncation.

@douwe Thanks for reviewing this fix for the issue mentioned in https://dev.gitlab.org/gitlab/gitlabhq/issues/2314#note_48539.

**BEFORE**

![before](https://dev.gitlab.org/gitlab/gitlabhq/uploads/4ffb35f77155a92c64cfe36c66987eb0/before.gif)

**AFTER**

![after](https://dev.gitlab.org/gitlab/gitlabhq/uploads/2bb2d0c4a93164f6d47e24cdcb9a106b/after.gif)

See merge request !1867