tests: stabilize for recent wcache changes This goes with 47e3f554df35::d5622dfe4ba3. I'm not sure if it was really expected that there would be no wcache directory if neither execbit nor symlink is supported.

extdiff: avoid double backslashes in the displayed tool path on Windows This shows the tool path in the help, and changed in 67b180c0e263. uirepr() already does the same thing, but that undoes the mangling in its call to repr().

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('.').

vfs: add option to not create parent directories implicitly In blackbox, we don't want to create a ".hg" directory by mistake. This provides a race-safe option to achieve that.

repo: add a `wcachevfs` to access the `.hg/wcache/` directory This wvfs will allow us to migrate various cache to the new `wcache` directory. Helping with cache issues with "share".