persistent-nodemap: add test case reproducing a Rust panic stable

persistent-nodemap: add test case reproducing a Rust panic

Thanks to Mitchell Plamann for reporting this bug.
The next commit fixes it.

Differential Revision: https://phab.mercurial-scm.org/D9900

--- a/tests/test-persistent-nodemap.t	Sun Jan 24 19:14:50 2021 -0500
+++ b/tests/test-persistent-nodemap.t	Thu Jan 28 13:25:37 2021 +0100
@@ -31,6 +31,15 @@
 
 #endif
 
+#if rust
+
+Reported bug: some Rust code panics when handling the null revision
+
+  $ hg log -r 00000000 2>&1 | grep panicked
+  thread '<unnamed>' panicked at 'called `Option::unwrap()` on a `None` value', hg-cpython/src/revlog.rs:* (glob)
+
+#endif
+
 
   $ hg debugformat
   format-variant     repo