git stat: the beginning of "status that is not a dry-run of commit"

Tentatively add "git stat" as a new command.

This is not "preview of commit with the same arguments"; the path parameters
are not paths to be added to the pristine index (aka "--only" option), but
are taken as pathspecs to limit the output.  Later in 1.7.0 release, it will
take over "git status".

Signed-off-by: Junio C Hamano <gitster@pobox.com>