[PATCH 6 of 9 "] discovery: simplify the building of the children mapping

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at octobus.net>
# Date 1551796794 -3600
#      Tue Mar 05 15:39:54 2019 +0100
# Node ID 1d2a2e4a255fcc8e5713232f5f80858a7b3793e1
# Parent  3f4f517d5ed962a0c2aef538ac81bfab28341355
# EXP-Topic discovery-speedup
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 1d2a2e4a255f
discovery: simplify the building of the children mapping

Since we only care about the revisions inside the set we are sampling, we can
use simpler code (and probably sightly faster).

diff --git a/mercurial/setdiscovery.py b/mercurial/setdiscovery.py
--- a/mercurial/setdiscovery.py
+++ b/mercurial/setdiscovery.py
@@ -219,13 +219,13 @@ class partialdiscovery(object):
         for rev in sorted(revs):
             # Always ensure revision has an entry so we don't need to worry
             # about missing keys.
-            children.setdefault(rev, [])
-
+            children[rev] = []
             for prev in parentrevs(rev):
                 if prev == nullrev:
                     continue
-
-                children.setdefault(prev, []).append(rev)
+                c = children.get(prev)
+                if c is not None:
+                    c.append(rev)
 
         _updatesample(revs, revsroots, sample, children.__getitem__)
         assert sample