Legit status deleted file support, minor UI overhaul

[jfaz1]

This PR adds support for deleted files in Legit. It also incidentally overhauls a bit of the status UI.

Here's the before/after:

Change summary:

• Deleted files now show up under staged/unstaged
• Untracked files now have a diff preview
• Number of files per section are now listed
• Modifications now have their type listed to the left, similar to Magit (created/modified/deleted)

Deleted files don't have a diff preview as of right now, instead there's just a message stating the file has been deleted. Should probably be revisited at some point to have the deleted file's contents show up.

Also, I'm unfamiliar with Mercurial and Fossil. I attempted to keep it compatible, but I was unable to test properly on Fossil (couldn't commit with/without my changes). Mercurial is mostly fine, the only gripe is that while deleted files show fine in the UI, trying to stage them does nothing. If I had to guess the issue is probably here:

(defun hg-stage (file)
  (run-hg (list "add" file)))

Feel free to suggest or just make any changes!

[vindarel]

LGTM

I was delighted to be able to pick Lem sources to start legit and I delighted to see another contributor can pick up my legit code.

Thanks for doing it.

[vindarel]

minor but necessary: please look at porcelain.lisp and legit.lisp top-level comments and at the README for the TODOs and update what's now done.

[jfaz1]

@vindarel Thanks for the kind words! I removed the missing file TODO from porcelain.lisp. I think the legit.lisp feature list looks fine as is, since I just added support for staging deleted files, and staging is already there.

[mmontone]

This is nice. A cool next step could be to implement a legit-log command that opens a buffer and lists commits.

[vindarel]

This is nice. A cool next step could be to implement a legit-log command that opens a buffer and lists commits.

absolutely.