setdiscovery: directly run '_updatesample'

The heads and exponential sample are going to end up in the same set
before any extra processing happens. We simplify the code by directly
updating a set with heads.

Changes in the order the set is built lead to small changes in the random
sampling output. But after double checking, I can confirm the input data to
the random sampling is consistent.

mercurial/setdiscovery.py

@@ -108,7 +108,7 b' def _takefullsample(dag, nodes, size):'
     always = dag.headsetofconnecteds(nodes)
     if size <= len(always):
         return always
-    sample = set()
+    sample = always
     # update from heads
     _updatesample(dag, nodes, sample, always)
     # update from roots

tests/test-setdiscovery.t

@@ -326,7 +326,7 b' One with >200 heads, which used to use u'
   query 6; still undecided: 540, sample size is: 200
   sampling from both directions
   searching: 7 queries
-  query 7; still undecided: 37, sample size is: 37
+  query 7; still undecided: 46, sample size is: 46
   7 total queries
   common heads: 3ee37d65064a