tag: disallow tagging the working directory

It's kinda silly, but a clear error message is better than a stacktrace about
subscripting `None` when trying to generate the default commit message.  I'm
surprised that `.revsingle(..).node()` returns None instead of `nodemod.wdirid`,
but now there's a test to catch if this changes.

mercurial/commands.py

@@ -7472,6 +7472,9 b' def tag(ui, repo, name1, *names, **opts)'
                 )
         node = logcmdutil.revsingle(repo, rev_).node()
 
+        if node is None:
+            raise error.InputError(_(b"cannot tag working directory"))
+
         if not message:
             # we don't translate commit messages
             message = b'Added tag %s for changeset %s' % (

tests/test-tag.t

@@ -412,6 +412,10 b' tagging on null rev'
   abort: cannot tag null revision
   [10]
 
+  $ hg tag -R empty -r "wdir()" -f wdirtag
+  abort: cannot tag working directory
+  [10]
+
 issue5539: pruned tags do not appear in .hgtags
 
   $ cat >> $HGRCPATH << EOF