tests: demonstrate crash caused by pinning of non-existent mergestate node If the mergestate somehow contains a reference to a non-existent node, it causes most commands to crash since aaeccdb6e654 (repoview: pin revisions for `local` and `other` when a merge is active, 2020-09-04). This patch adds a test showing that. Differential Revision: https://phab.mercurial-scm.org/D9077