Repository Page - Truncate last commit in table header

Resizing the browser causes the Last Commit in the table header to break lines. This causes some readability issues and doesn't allow the user to visually scan the information in the table as easily.

Make sure that the last commit is truncated to the appropriate size at all browser widths.

Original:

Screen_Shot_2016-07-07_at_1.40.26_PM

Proposed:

Screen_Shot_2016-07-07_at_1.42.05_PM

Central Issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/19610