cmdutil: changed walkchangerevs to use spanset instead of baseset

Using a spanset takes almost no memory at all. A baseset builds the entire
list in memory and is much slower for methods like __contains__.

mercurial/cmdutil.py

@@ -1296,7 +1296,7 b' def walkchangerevs(repo, match, opts, pr'
     elif follow:
         revs = repo.revs('reverse(:.)')
     else:
-        revs = revset.baseset(repo)
+        revs = revset.spanset(repo)
         revs.reverse()
     if not revs:
         return []