git-rebase: Add --stat and --no-stat for producing diffstat on rebase

git-rebase: Add --stat and --no-stat for producing diffstat on rebase

The behavior of --verbose is unchanged, but uses a different state
variable internally, so that the meaning of verbose output may be
expanded without affecting the diffstat. This is also reflected in
the documentation.

The configuration option rebase.stat works the same was as merg.stat,
but the default is currently false.

Signed-off-by: Tor Arne Vestbø <torarnv@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>