Reduce width of parallel diffs

At 50% width the alignment is off on parallel diffs (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3785/diffs and view in Side-by-Side)  
Reducing width lines them up:

<img src="/uploads/5775d35b0e86397abd10db6195a8a54d/Screen_Shot_2016-04-22_at_2.19.21_PM.png" width="400px">

Also shifted comment button over to center it properly:  
<img src="/uploads/c2468e15bb65523464a941001c3d7298/Screen_Shot_2016-04-22_at_2.24.33_PM.png" width="200px">

See merge request !3891