difftool/mergetool: make the form of yes/no questions consistent

difftool/mergetool: make the form of yes/no questions consistent

Every yes/no question in difftool/mergetool scripts has slightly
different form, and none of them is consistent with the form git
itself uses.

Make the form of all the questions consistent with the form used
by git.

Reviewed-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Nikola Forró <nforro@redhat.com>
Acked-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>