Change definition of path equality

[Mats-SX]

Undirected matching on self-relationships (aka loops) is tricky. #162 makes an effort to try and make explicit the intended semantics. This leaves paths in an ill-defined state, however. Consider the graph with one node and one relationship (to itself), and the following query:

MATCH p = ()--()
RETURN p

#162 states that only one row is to be returned (there's only one combination of node-rel-node available in the data graph). However, if paths are supposed to capture the direction in which they traverse relationships, then which path should be returned by this query: the one that traverses the relationship outgoing, or the one that does it incoming? It seems ill-defined.

In no other circumstance other than a self-relationship will the traversed direction matter.

Same idea as in #160

[thobe]

This feels wrong to me.

[thobe]

I think there are more scenarios that should be considered, for completeness:

Given an empty graph
And having executed:
  """
  CREATE (a)-[:X]->(a)
  """
When executing query:
  """
  MATCH p=()-->()
  MATCH q=()--()
  RETURN p = q
  """
Then the result should be:
  | p = q |
  |  ???  |

Where the result could be either:

  | p = q |
  | true  |

or:

  | p = q |
  | true  |
  | false |

(without order)

and

Given an empty graph
And having executed:
  """
  CREATE (a)-[:X]->(a)
  """
When executing query:
  """
  MATCH p=()<--()
  MATCH q=()--()
  RETURN p = q
  """
Then the result should be:
  | p = q |
  |  ???  |

With the same possible results as above.