Resolve code duplication in `commit_item.vue` and `last_commit.vue`

Problem to solve

There is a bit of code duplication between these two components:

• app/assets/javascripts/diffs/components/commit_item.vue
• app/assets/javascripts/repository/components/last_commit.vue

Let's try to fix this tech debt 💪

Strategy

So that we don't introduce any regressions, it's probably wise to create a Jest snapshot test of the two components (or parent components).