This option can be used to move one or multiple variables from one
place to another, e.g. to move some aliases from repo config to user
config, or from repo config to per-worktree config.

This will be useful for moving config variables around, especially when
enabling or disabling extensions.worktreeConfig (see the update in
git-worktree.txt for a real example).

The implementation is definitely not the best. We could lock both
source and destination before the move, and perhaps edit these files
just once instead of once per key. But it adds a lot more complication
to config write code. Let's stay with something simple. We can
optimize it later.