theme comparison tool: use "git worktree" to build themes

[mgeier]

This is not meant to be merged!

This is just a proof of concept, to be able to actually try if the "git worktree" approach could work.

Of course a lot of error checking, comments and other details are missing.

The goal was to implement the essential functionality with as little code as possible.

The version in #379 has many more features, but I just wanted to try the basic stuff.

[s-weigand]

Just tried it and it works nicely 😄

But I found 2 Problems:

1. If the user removed devtools/_worktree manually, git still knows it and won't create a new worktree.
   This can be solved with (the replace is to support windows):

if not WORKTREE_DIR.exists():
    if str(WORKTREE_DIR).replace("\\", "/") in repo.git.worktree("list"):
        repo.git.worktree("remove", WORKTREE_DIR)
    repo.git.worktree("add", WORKTREE_DIR, "--detach")

2. New unstaged files are ignored .

[mgeier]

Thanks for trying this out!

ad 1: I've also seen this problem in the meantime. Your solution should work fine, but probably we could use as_posix() to avoid the manual replace?
We could also try git worktree prune instead of explicitly removing it?

ad 2: I know. As mentioned in #379 (comment), --include-untracked doesn't seem to be available here.
I would just keep this as a known limitation (unless you know a simple solution), I don't think it is a big deal.

Anyway, I just wanted to see here if what I suggested in #379 would actually work and how much code it would need.

Obviously this is much shorter than #379, but of course there are several features missing here.

Do you want to try to make your solution in #379 shorter (or at least simpler) or probably use a few things from here over there?

[s-weigand]

Using as_posix() sounds good, as you saw in my code I use os.path all the time, guess I should use pathlib more often.

I will refactor #379 and use some of your code.

[mgeier]

I've squashed the commits and made this ready for review.

@s-weigand I've tried to keep your authorship for the documentation.

I think it works really well, except the custom-formats page doesn't seem to be cached successfully (but I guess that's not the fault of this PR).