revset: follow(nosuchfile) should give an empty set (issue3114)

mercurial/revset.py

@@ -434,8 +434,11 b' def follow(repo, subset, x):'
     p = repo['.'].rev()
     if l:
         x = getstring(l[0], _("follow expected a filename"))
+        if x in repo['.']:
         s = set(ctx.rev() for ctx in repo['.'][x].ancestors())
     else:
+            return []
+    else:
         s = set(repo.changelog.ancestors(p))
 
     s |= set([p])