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.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'oc/mergetools-beyondcompare' into pu

* oc/mergetools-beyondcompare:
  mergetool: rename bc3 to bc