setdiscovery: avoid calling any sample building if the undecided set is small

If the length of undecided is smaller than the sample size, we can just request
information for all of them.

This conditional was previously handled by '_setupsample'. But '_setupsample' is
in my opinion a problematic function with blurry semantics. Having this
conditional explicitly earlier makes the code more explicit and moves us closer
to removing this '_setupsample' function.

mercurial/setdiscovery.py

@@ -74,8 +74,6 b' def _updatesample(dag, nodes, sample, al'
                 visit.append(p)
 
 def _setupsample(dag, nodes, size):
-    if len(nodes) <= size:
-        return set(nodes), None, 0
     always = dag.headsetofconnecteds(nodes)
     desiredlen = size - len(always)
     if desiredlen <= 0:
@@ -205,6 +203,9 b' def findcommonheads(ui, local, remote,'
             ui.debug("taking quick initial sample\n")
             samplefunc = _takequicksample
             targetsize = initialsamplesize
+        if len(undecided) < targetsize:
+            sample = list(undecided)
+        else:
         sample = samplefunc(dag, undecided, targetsize)
         sample = _limitsample(sample, targetsize)