Refactor to remove duplicate code caused by `CommitWidget` and `CommitItem`

(Identified in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21784#note_102016371)

Description

To resolve #49329 (closed), we've created Vue components that clone:

• views/projects/merge_requests/diffs/_commit_widget.html.haml
• views/projects/commits/_commit.html.haml

Let's refactor the existing pages to clean up this duplicate code.

/cc @andr3