Re: [PATCH v3 4/6] worktree: new config file hierarchy

> ($C stands for $GIT_COMMON_DIR for the rest of the message)
>
> In main worktree, we read these config files in this order:
>
> 1) system config
> 2) XDG config
> 3) user config
> 4) $GIT_DIR/config
>
> Currently linked worktrees share the same config file at step 4 with
> main worktree. The problem is, not all config variables can be
> shared. We need per-repo vars and per-worktree ones.
>
> With this patch, since worktree v1, the repo config file (or worktree
> config file in multi worktree context) is no longer shared. Main
> worktree reads $C/config.  Linked worktrees read $C/worktrees/xxx/config
> and a new file, $C/worktrees/config. Sharing is done via this new
> file. The read hierarchy for a worktree becomes
>
> 1) system config
> 2) XDG config
> 3) user config
> 4) $C/common/config
> 5) $C/worktrees/xxx/config (or $C/config for main worktree)
>
> Compare to an alternative scheme where $C/config contains both shared
> variables and main-worktree-only ones, this is a cleaner design.
>
> * We do not have to check every single variable name to see if it's
>   shared or per-worktree when reading config files.
>
> * We do not enforce any particular variable split. If a variable
>   is in $C/worktrees/config, it is shared. Putting core.worktree in
>   $C/worktrees/config is punished the same way the variable is put in
>   $HOME/.gitconfig, for example.
>
> * We will provide a new "git config --repo" to access this new config
>   file. In single-worktree context, or worktree v0, --repo is an alias
>   of --local.
>
> There is one problem though. We store worktree version in config file
> and expect that all worktrees must share the same version (i.e. read
> the same config file). But the share-ness of per-repo config files is
> changed based on worktree version. Where do we put extensions.worktree
> then?