This prepares 'git-difftool' and its documentation for
mainstream use.
'git-difftool-helper' became 'git-difftool--helper'
since users should not use it directly.
'git-difftool' was added to the list of commands as
an ancillaryinterrogator.
Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
