packaging: avoid running bare "make install" in debian/rules

We change the "override_dh_install" target to "override_dh_auto_install"
in debian/rules (see dh_auto_install(1) for details). This is because we
do not want "make install" to be run automatically as we need the
--install-layout=deb of "setup.py install". Otherwise, files end up in
$DESTDIR/usr/local. At the moment, this is not a problem since files
installed in debian/tmp (the default location) are not packed into
binary packages (because there are debian/mercurial and
debian/mercurial-common directories). This is cleaner to avoid running
make more than needed.

contrib/packaging/debian/rules

@@ -25,7 +25,7 b' override_dh_auto_build:'
 	$(MAKE) all
 	$(MAKE) -C contrib/chg all
 
-override_dh_install:
+override_dh_auto_install:
 	python3 setup.py install --root "$(CURDIR)"/debian/mercurial --install-layout=deb
 	# chg
 	make -C contrib/chg \