Add ability to stage individual files to commit in multi-file editor

It's easy to end up editing many files, but only wanting to commit changes to a few of them, to keep each commit small or because some changes should be discarded. For this reason git clients allow individual files to be changed, even individual changed lines. We should support file level staging as soon as possible.

Design

details:

• icons are greyscale unless a change is present in that category

• Where to find the icons: Search staged and unstaged on gitlab-svgs.

Note: the unstaged icon might look slightly different from mockups

• Actions on hover (stage, discard) (unstage)