contrib: add a helper script that help to build interesting repositories The script is dedicated to building a couple of repositories that should be interesting to run discovery from one another. It seems a common enough need to contribute it upstream.

py3: listify filter() to call len() on it Differential Revision: https://phab.mercurial-scm.org/D5354

loggingutil: document openlogfile() This function will be used later for command-server logging.

loggingutil: extract openlogfile() and proxylogger to new module This module isn't placed under the "utils" package since it needs "ui" to process things. It's called "loggingutil", not "logutil" because the word "log" is too obscure in our codebase.

blackbox: pass in options to _openlogfile() as arguments This prepares for extracting utility function from the blackbox module.

blackbox: just try writing to repo.vfs and update lastlogger on success This is simpler and more robust. Before, an empty ".hg" directory would be created if it's removed after checking vfs.isdir('.').