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
local test passed
addgroupcopy test passed
clone test passed
lowlevelcopy test passed
slicing test passed
issnapshot test passed
findsnapshot test passed