Correctly find last known blob for file deleted in MR.

Fixes #3092 (closed).

When building a new MR, @merge_request.commits.last would fail because this delegates to merge_request_diff which is still nil at that point. I fixed that, and changed some of the logic because showing deleted blob contents didn't previously work for the Compare page, and the UI would show the wrong commit sha for "View File @...".

mentioned in issue #3078 (closed)

Impressive. Glad you found the problem @DouweM

Status changed to merged

mentioned in issue #427 (closed)

Picked into 8-1-stable.

mentioned in commit 44d35a82

mentioned in commit f16bfa40