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>
Merge branch 'oc/mergetools-beyondcompare' into jch

* oc/mergetools-beyondcompare:
  mergetool: rename bc3 to bc
diff -B -M: fix output for "copy and then rewrite" case

Starting from a single file, A, if you create B as a copy of A (and
possibly make some edit) and then make extensive change to A, you
will see:

    $ git diff -C --name-status
    C89    A    B
    M      A

which is expected.  However, if you ask the same question in a
different way, you see this:

    $ git diff -B -M --name-status
    R89    A    B
    M100   A

telling us that A was rename-edited into B (as if "A will no longer
exist as the result") and at the same time A itself was extensively
edited.

In this case, because the resulting tree still does have file A
(even if it has contents vastly different from the original), we
should use "C"opy, not "R"ename, to avoid hinting that A somehow
goes away.

Two existing tests were depending on the wrong behaviour, and fixed.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jc/diff-b-m' into jch

* jc/diff-b-m:
  diff -B -M: fix output for "copy and then rewrite" case