mergetool: rename bc3 to bc

Beyond Compare version 4 works the same way as version 3, so rename
the existing "bc3" adaptor to just "bc", while keeping "bc3" as a
backward compatible wrapper.

Noticed-by: Olivier Croquette <ocroquette@free.fr>
Helped-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>