chg: format code by clang-format version 11.0.1-+rc1-1 test-check-clang-format.t started failing on my Debian sid environment, and new style looks slightly better. So let's bump the required clang-format version to 11.

test-check-interface: do not expect bare "python" executable exists The test would crash if python executable had version suffix.

procutil: assign pseudo file object if sys.stdout/stderr is missing This basically simulates the Python 2 behavior. If libc stdio were used, these file objects would be available and raise EBADF. There is subtle difference between py2 and py3, but I think py3 behavior (i.e. exit 255) is more correct. "if" conditions are adjust so that they look similar to dispatch.initstdio().

procutil: introduce pseudo file object that just raises EBADF This should be safer than closing underlying fd as the fd may be reused. On Python 2, closed sys.stdin could be redirected to a random file having fd=0, but we'd be better not copying this behavior. Only readinto() and write() are implemented according to the following ABC table. fileno() is not implemented since fd=0/1/2 may be assigned later to other files. https://docs.python.org/3/library/io.html#class-hierarchy

dispatch: gate against missing stdout/stderr We do need procutil.stdin/stdout/stderr, but we don't care much for sys.std*. Let's leave them be None as it is the Python 3 way.

dispatch: remove stale comment about fdopen()-ed stdio On Python 3, stdout is just wrapped by LineBufferedWrapper.

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

mergetools: add support for 32-bit BeyondCompare 3 on 64-bit Windows Again, marginal usefulness, but other tools have this config and it eliminates a diff from the TortoiseHg file. Differential Revision: https://phab.mercurial-scm.org/D9634

mergetools: enable the `checkconflicts` option for Araxis Merge This must have been a typo. This matches the config read in filemerge, as well as the TortoiseHg config file. Differential Revision: https://phab.mercurial-scm.org/D9633

mergetools: sync up comments with the TortoiseHg copy of the config file It's of limited value, but a step towards making the diff between the two less noisy. Differential Revision: https://phab.mercurial-scm.org/D9632