This will make Mercurial be able to find third-party extensions installed with
Guix, without having to set PYTHONPATH.
* gnu/packages/patches/mercurial-hg-extension-path.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register the patch.
* gnu/packages/version-control.scm (mercurial)[origin](patches): Apply the
patch.
[native-search-paths]: Add HGEXTENSIONPATH.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
