setdiscovery: delay sample building calls to gather them in a single place

Some of the logic around sample building is duplicated in the sample builders,
it would clean up thing to extract it in the top function, but this requires
all codes to be in the same place.

This changeset mostly exists to make the next one more clear.

mercurial/setdiscovery.py

@@ -198,13 +198,14 b' def findcommonheads(ui, local, remote,'
                 ui.note(_("sampling from both directions\n"))
             else:
                 ui.debug("taking initial sample\n")
-            sample = _takefullsample(dag, undecided, size=fullsamplesize)
+            samplefunc = _takefullsample
             targetsize = fullsamplesize
         else:
             # use even cheaper initial sample
             ui.debug("taking quick initial sample\n")
-            sample = _takequicksample(dag, undecided, size=initialsamplesize)
+            samplefunc = _takequicksample
             targetsize = initialsamplesize
+        sample = samplefunc(dag, undecided, targetsize)
         sample = _limitsample(sample, targetsize)
 
         roundtrips += 1