configitems: register server.zstdlevel Somehow, I managed to trigger a devel-warn running `hg serve` outside the test suite on one of the repos generated by running test-lfs-serve-access.t --keep. (I'm not hitting it now after doing a `make local`.) The only reference to this in all of the history is the help text added in e75463e3179f, and the translations. (It looks like the string is built dynamically with '%slevel', which is probably how this was missed. I wonder if this isn't getting routed to error.log for some reason.) In any event, server.zliblevel is registered, and that's required to pick up the documented default.

--- a/mercurial/configitems.py	Sat Apr 14 00:13:08 2018 -0700
+++ b/mercurial/configitems.py	Sun Apr 15 22:28:03 2018 -0400
@@ -944,6 +944,9 @@
 coreconfigitem('server', 'zliblevel',
     default=-1,
 )
+coreconfigitem('server', 'zstdlevel',
+    default=3,
+)
 coreconfigitem('share', 'pool',
     default=None,
 )