setdiscovery: limit the size of the initial sample (issue4411)

The set discovery start by sending a "known" command with all local heads. When
the number of local heads is massive (eg: using hidden changesets) such request
becomes too large. This lead to 414 error over http, aborting the whole
process.

We limit the size of the sample used by the first query to fix this.

The test are impacted because they do test massive number of heads. But they do
not test it over real world http setup.

mercurial/setdiscovery.py

@@ -133,7 +133,7 b' def findcommonheads(ui, local, remote,'
     ui.debug("query 1; heads\n")
     roundtrips += 1
     ownheads = dag.heads()
-    sample = ownheads
+    sample = _limitsample(ownheads, initialsamplesize)
     if remote.local():
         # stopgap until we have a proper localpeer that supports batch()
         srvheadhashes = remote.heads()

tests/test-setdiscovery.t

@@ -311,16 +311,16 b' One with >200 heads, which used to use u'
   searching for changes
   taking quick initial sample
   searching: 2 queries
-  query 2; still undecided: 1080, sample size is: 260
+  query 2; still undecided: 1240, sample size is: 260
   sampling from both directions
   searching: 3 queries
-  query 3; still undecided: 820, sample size is: 260
+  query 3; still undecided: 980, sample size is: 260
   sampling from both directions
   searching: 4 queries
-  query 4; still undecided: 560, sample size is: 260
+  query 4; still undecided: 720, sample size is: 260
   sampling from both directions
   searching: 5 queries
-  query 5; still undecided: 300, sample size is: 200
+  query 5; still undecided: 460, sample size is: 200
   5 total queries
   common heads: 3ee37d65064a