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

rebase: fix dir/file conflict detection when using in-mem merge Differential Revision: https://phab.mercurial-scm.org/D5360

tests: show that in-mem rebase does not find path dir/file conflicts Differential Revision: https://phab.mercurial-scm.org/D5359

extdiff: register the configuration generated commands with a help category Otherwise, 'extdiff' shows up under file management and the rest of the commands are at the bottom under 'Uncategorized'.