tag: disallow tagging the working directory stable

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.

--- a/mercurial/commands.py	Thu Feb 16 20:33:14 2023 +0100
+++ b/mercurial/commands.py	Tue Feb 14 11:56:02 2023 -0500
@@ -7472,6 +7472,9 @@
                 )
         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' % (

--- a/tests/test-tag.t	Thu Feb 16 20:33:14 2023 +0100
+++ b/tests/test-tag.t	Tue Feb 14 11:56:02 2023 -0500
@@ -412,6 +412,10 @@
   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