Add 'stg cp' command.

Add 'stg cp' command.

Supports copying several files and dirs into existing dir, as well as
copying single file or dir with a new name.

In the case of directory copying, only copies files known to git (ie,
no generated files).  Should maybe add --all as alternative.

Does not check whether we're going to lose data (overwriting any
modified file, or any file not tracked by git).  This should probably
be done first, before doing any real change (preparing the changes
in-memory, then do the work if OK).  Will give us --dry-run at no
additional cost.

Signed-off-by: Yann Dirson <ydirson@altern.org>