Add "null" pseudo-tag pointing to nullid

mercurial/commands.py

@@ -2189,7 +2189,7 b' def tag(ui, repo, name, rev_=None, **opt'
     necessary.  The file '.hg/localtags' is used for local tags (not
     shared among repositories).
     """
-    if name in ['tip', '.']:
+    if name in ['tip', '.', 'null']:
         raise util.Abort(_("the name '%s' is reserved") % name)
     if rev_ is not None:
         ui.warn(_("use of 'hg tag NAME [REV]' is deprecated, "

mercurial/localrepo.py

@@ -376,6 +376,8 b' class localrepository(repo.repository):'
             key = self.dirstate.parents()[0]
             if key == nullid:
                 raise repo.RepoError(_("no revision checked out"))
+        elif key == 'null':
+            return nullid
         n = self.changelog._match(key)
         if n:
             return n