I think having both --git-dir and --work-tree options is necessary to cover the case where the .git directory isn't located within the work tree or isn't named .git.

It seems like we should have tests for that but I guess we don't since the tests passed with this change.

the .git being a file in place of a directory is exactly the reason I started down the rabbit hole, I believe it currently has bugs, for both submodules and worktrees, opening a Git and trying to checkout a new branch fails. Even if the submodules code is to be fixed it should be duplicated or moved to the initialize function.
I'm in too deep and I'm a bit lost for a few days already...

The two switches need to be amended differently for each case and I'm yet to fully get exactly how they work, couldn't find much documented to explain what they mean.
I also get into weird states where the change tree doesn't match the preped files or a .lock file missing issue...
So the -C is close to an actual change directory, which actually works in place of the two switches for all cases. git CLI just figures them alone and the rest of the stuff.. you may still override them even after a -C when they would be relative to that (assuming relative paths are used over full ones)

I'm happy to add tests for these, due they currently fail for other stuff...
If you want to depend this change merge on added tests for submodule and worktree, that's fine.

So this is a submodule test it fails for both master and this PR (#487) 😭
#488 (against master)
hatkyinc2#1 (against this PR)

But my conclusions from it are:

• This code is probably not useful https://github.com/ruby-git/ruby-git/blob/master/lib/git/base.rb#L58-L65
• The separation of --git-dir and --work-tree over -C doesn't help currently (nor does this patch actually fix things, but rather simple than complicated if it's the same result?)

Unless my test is just broken in a way I can't see at the moment 🙈

This is a test for worktree new branch checkout, same error as with the submodule
#489

I think at this point I need help.
Like I don't get it, if I do exactly as the test and the command listed on the terminal it works
But obviously this doesn't work.

It's probably something super silly, that's how it usually works 😞

@hatkyinc2 I was wondering if you could take a look at #490. In addition to fixing a bug (how the default for index is calculated) it also adds a test for when git-dir is in a non-standard place relative to work-tree. This test should fail with the change you are proposing in this PR.

Closing this for the reasons stated in the comments above. It comes down to the matter that sometimes the .git file/directory is not within the working copy.