message: Add Beyond Compare 4 to mergetools
  BC4 executable: 'bcompare' (Linux), 'BCompare.exe' (Windows)
diff: [mergetool "bc"]
  cmd = bcomp \"$LOCAL\" \"$REMOTE\" \"$BASE\" -mergeoutput=\"$MERGED\"
  +[difftool "bc4"]
  + cmd = bcompare \"$LOCAL\" \"$REMOTE\"
  +
  +[mergetool "bc4"]
  + cmd = bcompare \"$LOCAL\" \"$REMOTE\" \"$BASE\" -mergeoutput=\"$MERGED\"
  +
  [difftool "codecompare"]
  cmd = CodeCompare \"$LOCAL\" \"$REMOTE\"
  