persistent-nodemap: document the `devel.persistent-nodemap` option
What it does is not obvious.
Differential Revision: https://phab.mercurial-scm.org/D9753
--- a/mercurial/configitems.py Fri Jan 15 23:47:16 2021 +0100
+++ b/mercurial/configitems.py Thu Jan 14 00:33:40 2021 +0100
@@ -635,6 +635,8 @@
b'legacy.exchange',
default=list,
)
+# When True, revlogs use a special reference version of the nodemap, that is not
+# performant but is "known" to behave properly.
coreconfigitem(
b'devel',
b'persistent-nodemap',