config: add --move-to

config: add --move-to

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 when
extensions.worktreeConfig is enabled. E.g.

    git config --local --move-to --worktree core.worktree
    git config --local --move-glob-to --worktree 'submodule.*.*'

The implementation is definitely not the best. We could for example
lock both source and destination files before doing any update, and
perhaps edit these files just once instead of once per key.

But it adds a lot more complication to config update code. Let's stay
with something simple for now. It's not worse than scripting using
"git config". Optimization could be done later.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>