revsets: define a none() revset

This can be useful for default values in corp-wide configuration
(e.g. for commits to hide, which defaults to none()), and perhaps in
scripts. I named it none() rather than empty() to match all().

Differential Revision: https://phab.mercurial-scm.org/D3713

mercurial/revset.py

@@ -1346,6 +1346,14 b' def node_(repo, subset, x):'
     result = baseset([rn])
     return result & subset
 
+@predicate('none()', safe=True)
+def none(repo, subset, x):
+    """No changesets.
+    """
+    # i18n: "none" is a keyword
+    getargs(x, 0, 0, _("none takes no arguments"))
+    return baseset()
+
 @predicate('obsolete()', safe=True)
 def obsolete(repo, subset, x):
     """Mutable changeset with a newer version."""

tests/test-revset2.t

@@ -584,6 +584,9 b' test empty string'
   hg: parse error: empty string is not a valid revision
   [255]
 
+test empty revset
+  $ hg log 'none()'
+
 we can use patterns when searching for tags
 
   $ log 'tag("1..*")'