Add '.' as a shortcut revision name for the working directory parent.

mercurial/commands.py

@@ -2789,8 +2789,8 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 == "tip":
-        raise util.Abort(_("the name 'tip' is reserved"))
+    if name in ['tip', '.']:
+        raise util.Abort(_("the name '%s' is reserved") % name)
     if rev_ is not None:
         ui.warn(_("use of 'hg tag NAME [REV]' is deprecated, "
                   "please use 'hg tag [-r REV] NAME' instead\n"))

mercurial/localrepo.py

@@ -292,6 +292,10 b' class localrepository(repo.repository):'
         try:
             return self.tags()[key]
         except KeyError:
+            if key == '.':
+                key = self.dirstate.parents()[0]
+                if key == nullid:
+                    raise repo.RepoError(_("no revision checked out"))
             try:
                 return self.changelog.lookup(key)
             except: