revset-children: call 'getset' on a 'fullreposet'

Calling 'baseset(repo.changelog)' builds a list for all revisions in the
repo. And we already have the lazy and efficient 'fullreposet' class
for this purpose.

This gives us the usual benefits of the fullreposet:

revset) children(tip~100)
before) wall 0.007469 comb 0.010000 user 0.010000 sys 0.000000 (best of 338)
after)  wall 0.003356 comb 0.000000 user 0.000000 sys 0.000000 (best of 755)

mercurial/revset.py

@@ -572,7 +572,7 b' def children(repo, subset, x):'
     """``children(set)``
     Child changesets of changesets in set.
     """
-    s = getset(repo, baseset(repo), x)
+    s = getset(repo, fullreposet(repo), x)
     cs = _children(repo, subset, s)
     return subset & cs