Fix tab behavior on MergeRequests#new

Missed this one in 269aee79

Prior to this fix, clicking the Commits or Changes tab when creating a new MR would reload the page instead of display the tab contents.

mentioned in issue #1438 (closed)

Reassigned to @dzaporozhets

Ping @dzaporozhets

Milestone changed to 7.11

Status changed to merged

mentioned in commit 3ae9b696

mentioned in issue #1474 (closed)

mentioned in commit 20a66988