interdiff: teach show_interdiff() to indent interdiff

interdiff: teach show_interdiff() to indent interdiff

A future change will allow "git format-patch --interdiff=<prev> -1" to
insert an interdiff into the commentary section of the lone patch of a
1-patch series. However, to prevent the inserted interdiff from
confusing git-am, as well as human readers, it needs to be indented.
Therefore, teach show_interdiff() how to indent.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>