Migrate app/assets/javascripts/diffs/components/commit_item.vue input to GlFormCheckbox

In app/assets/javascripts/diffs/components/commit_item.vue, we still use a raw HTML tag <input that can be replaced by a GlFormCheckbox. Let's replace it!

See !90193 (comment 992405415)

app/assets/javascripts/diffs/components/commit_item.vue

<input
thought: This should probably be a GlFormCheckbox 🤔

Edited Jun 16, 2022 by David O'Regan