tests/test-wsgicgi.t

dirstate-v2: add check of parent/child nodes being related when writing

This stems from a corruption seen in a private repository. We're not sure
of the source of the corruption, and it's very possible that we're seeing
compounded effects of multiple writes on a corrupted dirstate.

Adding this check is not expensive in itself and large writes of the dirstate
are not common.

This change does not catch this problem at the root node, the next one will.

  $ hg init test
  $ cat >hgweb.cgi <<HGWEB
  > #!$PYTHON
  > from mercurial import demandimport; demandimport.enable()
  > from mercurial.hgweb import hgweb
  > from mercurial.hgweb import wsgicgi
  > application = hgweb(b"test", b"Empty test repository")
  > wsgicgi.launch(application)
  > HGWEB
  $ chmod 755 hgweb.cgi

Check that non-ASCII bytes roundtrip correctly.

  $ . "$TESTDIR/cgienv"
  $ PATH_INFO="/rev/$(python -c 'import sys; sys.stdout.buffer.write(b"\xe2\x80\x94")')"; export PATH_INFO
  $ QUERY_STRING="style=raw"; export QUERY_STRING
  $ "$PYTHON" ./hgweb.cgi | grep error
  error: unknown revision '\xe2\x80\x94' (esc)