tests/test-dirstate-nonnormalset.t

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

  $ cat >> $HGRCPATH << EOF
  > [ui]
  > logtemplate="{rev}:{node|short} ({phase}) [{tags} {bookmarks}] {desc|firstline}\n"
  > [extensions]
  > dirstateparanoidcheck = $TESTDIR/../contrib/dirstatenonnormalcheck.py
  > [experimental]
  > nonnormalparanoidcheck = True
  > [devel]
  > all-warnings=True
  > EOF
  $ mkcommit() {
  >    echo "$1" > "$1"
  >    hg add "$1"
  >    hg ci -m "add $1"
  > }

  $ hg init testrepo
  $ cd testrepo
  $ mkcommit a
  $ mkcommit b
  $ mkcommit c
  $ hg status