git aliases and GIT_PREFIX

Hello to everyone,

today I reported what I believed to be a bug on

  https://github.com/git-for-windows/git/issues/3496

and learned about GIT_DIR when working with aliases and git worktree.

It's annoying that GIT_DIR it is defined only if (as far as I've understood) working from a worktrees or submodule, as it does not seem to be related to those type of repositories.

This is also irritating because apparently working aliases breaks when being executed from those repositories.

I believe it would be better if GIT_DIR it's either always set or never (could someone enlighten me why the variable is needed in first place?).

If it is consistently set, it would make it much easier to notice that an alias is actually broken.

Best

Federico

PS: I'm not subscribed to the mailing list (yet), so please keep me in CC.