Firestore: fix watch against nested collections (issue #7515).

[Kyras]

Fix for issue mentioned in the issue #7515, by replacing the parent info of query target from the url to the root of the database, to actual query parent, stored in its parent info.

Collection query holds an _parent variable pointing to the Document/CollectionReference, which holds info about database path in the _parent_info(), which returns tuple containing:

1. url to the parent object
2. url to self

Thus, returning first value from _parent_info(), instead of url to root of database fixes the bug.

[tseaver]

Thank you for the patch! We need to add a unit test which actually exercises #7515 (creates a watch against a nested collection). The test should fail without this fix, and pass with it.

[tseaver]

Superseded by #7752.