mergetools: add BeyondCompare 4 for Windows This is a copy of the TortoiseHg configuration, but dropping the 3-way diff and dirdiff settings (since those aren't supported in core Mercurial), and swapping the 'parent1', 'parent2', and 'base' literals for `$labellocal`, `$labelother, and `$labelbase` respectively. (That functionality was apparently never ported to TortoiseHg.) Additionally, the single quotes were removed from around these arguments, since none of the other configs have them. Differential Revision: https://phab.mercurial-scm.org/D9635