Re: [PATCH v2] Documentation/git: fix stale "MULTIPLE CHECKOUT MODE" reference

> The other one is more heavy.  Do we even want to have and expose
> GIT_COMMON_DIR environment variable?
>
> The primary reason why we added GIT_DIR, GIT_OBJECT_DIRECTORY
> etc. in the early days of Git was because we didn't exactly know
> what kind of layout and flexibility was needed from "various SCMs
> that sit on top of Git core", and we wanted to make progress rapidly
> without making decisions back then.  But it is not 2005 anymore.
>
> Suppose a file "gitdir: /home/gitster/w/src/.git/worktrees/rerere"
> (call that $GIT_DIR) is there, and there is $GIT_DIR/commondir. Is
> there any valid reason why somebody may want to use only part of
> that arrangement and have a $GIT_COMMON_DIR that points at a place
> different from $GIT_DIR/commondir points at to override only a part
> of the setting?
>
> Or perhaps there is a plain vanilla $GIT_DIR that does not have
> $GIT_DIR/commondir.  Is there any valid reason why somebody may want
> to reuse only part of that directory as if it were a linked checkout
> and then use $GIT_COMMON_DIR to redirect the access to the meat of
> the repository elsewhere?
>
> The safety that comes from the primary checkout and the secondary
> checkouts all knowing everybody else is lost in such a use case,
> that is the whole point of adding this new feature.  The fact that
> $GIT_COMMON_DIR/worktrees/* and $GIT_DIR/commondir reference each
> other is what gives us object-prune-safety and multi-checkout-safety.
>
> Unless I am mistaken, I think a separate GIT_COMMON_DIR environment
> variable that can be tweaked by end-user is nothing but a source of
> future bugs and user confusion, without giving us any useful
> flexibility.