setdiscovery: extract sample limitation in a `_limitsample` function stable

setdiscovery: extract sample limitation in a `_limitsample` function

We need to reuse this logic for the initial query. We extract it in a function
to unsure sample limiting is applied consistently in all cases.

--- a/mercurial/setdiscovery.py	Fri Oct 24 17:24:46 2014 -0500
+++ b/mercurial/setdiscovery.py	Mon Oct 27 17:40:32 2014 +0100
@@ -105,14 +105,19 @@
     # update from roots
     _updatesample(dag.inverse(), nodes, sample, always)
     assert sample
-    if len(sample) > desiredlen:
-        sample = set(random.sample(sample, desiredlen))
-    elif len(sample) < desiredlen:
+    sample = _limitsample(sample, desiredlen)
+    if len(sample) < desiredlen:
         more = desiredlen - len(sample)
         sample.update(random.sample(list(nodes - sample - always), more))
     sample.update(always)
     return sample
 
+def _limitsample(sample, desiredlen):
+    """return a random subset of sample of at most desiredlen item"""
+    if len(sample) > desiredlen:
+        sample = set(random.sample(sample, desiredlen))
+    return sample
+
 def findcommonheads(ui, local, remote,
                     initialsamplesize=100,
                     fullsamplesize=200,