revset: drop support for '' as alias for '.'

Not marked BC because I think support for using '' on the CLI was
there by accident, and we don't seem to have documented it.

Differential Revision: https://phab.mercurial-scm.org/D3018

mercurial/revset.py

@@ -116,6 +116,8 b' def _sortedb(xs):'
 # operator methods
 
 def stringset(repo, subset, x, order):
+    if not x:
+        raise error.ParseError(_("empty string is not a valid revision"))
     x = scmutil.intrev(repo[x])
     if (x in subset
         or x == node.nullrev and isinstance(subset, fullreposet)):

tests/test-revset2.t

@@ -581,7 +581,8 b' test empty string'
   hg: parse error: empty query
   [255]
   $ log 'parents("")'
-  8
+  hg: parse error: empty string is not a valid revision
+  [255]
 
 we can use patterns when searching for tags