Correct (and clarify) tree-exclusion logic
Correct (and clarify) tree-exclusion logic

@mtholder correctly pointed out that my use of `and` here would clobber
unrelated decisions if they matched on *either* the study or tree ID!
The new version is hopefully a little clearer, in that it describes a
decision that matches both, then excludes it from the resulting list.

OpenTreeOfLife/phylesystem-api,OpenTreeOfLife/phylesystem-api,OpenTreeOfLife/phylesystem-api