Allow collapsing of commits

Description

When in the diff view for a branch it would be great to collapse the commits. This would be similar to collapsing an individual file. This would help when reviewing a diff for a branch with a long history.

Edit: I've added a link to the screen I'm mentioning. The example doesn't have a lot of commits but sometimes depending on the amount of changes the list of commits can be hundreds or more. In those scenarios it help to be able to collapse the commits list.

Proposal

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/compare/master...tz-ide-markdown-preview