Running Thicket.concat_thickets with calltree="intersection" produces empty query

[ilumsden]

When trying to merge 4 Thickets with RAJAPerf data with concat_thickets, Thicket errors with EmptyQuery because the query used in Thicket.intersection would produce an empty Thicket object. After reviewing the individual Thickets by hand, I found that their call trees are identical, so there is a bug in Thicket.intersection causing this error.

[michaelmckinsey1]

May be fixed by the changes to intersection in #182