revset-parentspec: 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) 100^1
before) wall 0.002694 comb 0.000000 user 0.000000 sys 0.000000 (best of 897)
after)  wall 0.000997 comb 0.000000 user 0.000000 sys 0.000000 (best of 2324)

revset) parents(100)^1
before) wall 0.003832 comb 0.000000 user 0.000000 sys 0.000000 (best of 587)
after)  wall 0.001034 comb 0.000000 user 0.000000 sys 0.000000 (best of 2309)

revset) (100^1)^1
before) wall 0.005616 comb 0.000000 user 0.000000 sys 0.000000 (best of 405)
after)  wall 0.001030 comb 0.000000 user 0.000000 sys 0.000000 (best of 2258)

mercurial/revset.py

@@ -1257,7 +1257,7 b' def parentspec(repo, subset, x, n):'
         raise error.ParseError(_("^ expects a number 0, 1, or 2"))
     ps = set()
     cl = repo.changelog
-    for r in getset(repo, baseset(cl), x):
+    for r in getset(repo, fullreposet(repo), x):
         if n == 0:
             ps.add(r)
         elif n == 1: