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

[Aaron Bentley]

Aaron Bentley has voted tweak.
Status is now: Conditionally approved
Comment:
Agreed.  (Actually, we could test that it has a select_conflicts method 
instead, but I can't imagine us ever reimplementing ConflictsList.) 
I'll tweak and merge.

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