mergetools/bc: add `bc4` to the alias list for Beyond Compare

mergetools/bc: add `bc4` to the alias list for Beyond Compare

As of 83bbf9b92ea8 (mergetool--lib: improve support for vimdiff-style
tool variants, 2020-07-29), we already list `bc` and `bc3` as aliases
for that mergetool/difftool.

However, the current Beyond Compare version is _4_, therefore the `bc4`
alias is missing from that list.

Most notably, this is the root cause of the breakage reported in
https://github.com/git-for-windows/git/issues/2893 where a
well-configured `bc4` difftool stopped working as of v2.29.0:
`setup_tool` would notice that after stripping off the trailing digit,
it finds a match in `mergetools/` (the `bc` file), source it, and then
the alias would not match the list offered by the `list_tool_variants`
function, and simply exit without doing anything, but pretending
success.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>