Add loading state when showing unchanged lines in MR
## Problem When showing unchanged lines in an MR diff, there's no indication of activity: ![CleanShot_20210513122118](https://gitlab.com/gitlab-org/gitlab-design/uploads/fe1237358d2415d1077a40518f7b8f3d/CleanShot_20210513122118.mp4) ## Proposal Consider replacing the button's icon with a loading spinner, matching how we display [loading buttons](https://gitlab-org.gitlab.io/gitlab-ui/?path=/story/base-button--loading-button).
issue