Fix issue 1393 - previous clause variables not seen with EXISTS (#1426) (#1429)

Fixed issue 1393 - Unexpected error when matching nodes that are
connected to nodes resulted from a previous MATCH clause.

Basically, variables from the previous clauses where not seen when
using the EXISTS clause for path pattern matches. This only affected
EXISTS and only for the EXISTS (anonymous_path) grammar component.

The original issue was only with vertices, but this also affected
edges. Both cases are now fixed.

Fixed 1 regression test that was incorrect.
Added regression tests.

Resolved -

Conflicts:
	src/backend/parser/cypher_clause.c

Author: jrgemignani