Re: GIT_DIR unset in vc-git.el

[Stefan Monnier]

> In vc-git.el, the function vc-git--call unsets GIT_DIR before calling
> git.  What is the reason for this?

When GIT_DIR is set (as in your case), it is usually valid only for some
of the files, and `vc-git` has no reason to believe that the GIT_DIR
that happened to be set in the process from which Emacs was started
applies to all the files that will be visited during this Emacs session.

`vc-region-history` says that the commit which introduced this GIT_DIR
override is:

    commit dc5e65b5deb2f5b67f6c3a06ae81c6b074bd4b56
    Author: Dmitry Gutov <dgutov@yandex.ru>
    Date:   Wed Jun 22 02:04:33 2016 +0300
    
        Unset GIT_DIR when calling Git commands
        
        * lisp/vc/vc-git.el (vc-git--call, vc-git-command):
        Unset GIT_DIR (bug#23769).

So you can check bug#23769 for further info.

>   (defun git-link-home ()
>     (interactive)
>     (let ((process-environment
>            `(,(format "GIT_DIR=%s" (expand-file-name "~/.home.git/"))
>              ,(format "GIT_WORK_TREE=%s" (expand-file-name "~/"))
>              ,@process-environment)))
>       (recursive-edit)))

Maybe you can save yourself the trouble.

    cd ~/.home.git; git worktree add ~ main

won't work because it will complain that ~ "already exists", but that's
just Git being too protective.  You can "do it by hand":

    cd ~/.home.git; git worktree add ~/dummy main
    mv ~/dummy/.git ~/.
    sed -i s/.dummy// ~/.home.git/worktrees/dummy/gitdir

and then your home's Git data will still be stored in that bare
`.home.git` but without having to mess with `GIT_DIR` and
`GIT_WORK_TREE`.  The only/main difference is that you'll now have
a `~/.git` file (which points at `~/.home.git/worktrees/dummy`).

> Is there any harm in changing vc-git--call to not do that?

In general there can be, but only in some specific circumstances, so you
can also opt to do that, of course.


        Stefan