nodemap: add a (python) index class for persistent nodemap testing Using the persistent nodemap require a compeling performance boost and an existing implementation. The benefit of the persistent nodemap for pure python code is unclear and we don't have a C implementation for it. Yet we would like to actually start testing it in more details and define an API for using that persistent nodemap. We introduce a new `devel` config option to use an index class dedicated to Nodemap Testing. This feature is "pure" only because having using a pure-python index with the `cext` policy proved more difficult than I would like. There is nothing going on in that class for now, but the coming changeset will change that. Differential Revision: https://phab.mercurial-scm.org/D7840

--- a/tests/test-persistent-nodemap.t	Wed Jan 15 15:47:59 2020 +0100
+++ b/tests/test-persistent-nodemap.t	Wed Jan 15 15:48:09 2020 +0100
@@ -8,6 +8,8 @@
   $ cat << EOF >> .hg/hgrc
   > [experimental]
   > exp-persistent-nodemap=yes
+  > [devel]
+  > persistent-nodemap=yes
   > EOF
   $ hg debugbuilddag .+5000
   $ f --size .hg/store/00changelog.n