Investigate performance regression in DocumentSourceGroup when allowDiskUse is true

In investigating BF-25112 a 20% regression was discovered after SERVER-63208 landed on master. An initial investigation ruled out $lookup in sbe from being the cause due to presence of an inner pipeline syntax. 

The investigator should profile DocumentSourceGroup with the following query 

db.foo.aggregate([{"$group": {"_id" : {"$sum" : ["$int", 1 ]}}])

With allowDiskUse defaulted to true and allowDiskUse=false.