[MERGE] Return ConflictsList() instead of [] from Tree.conflicts

[Jelmer Vernooij]

Jelmer Vernooij has voted tweak.
Status is now: Semi-approved
Comment:
I'd rather see assertIsInstance being used than assertTrue(isinstance()) 
because the resulting error message is clearer. Other than that, looks 
good.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1188239712.7894.9.camel%40nemo%3E