tag: move the prohibition of tagging the `null` rev up to the `wdir()` check

It makes sense to do these together, and avoid another revision lookup.

mercurial/commands.py

@@ -13,6 +13,7 b' import sys'
 from .i18n import _
 from .node import (
     hex,
+    nullid,
     nullrev,
     short,
     wdirrev,
@@ -7500,8 +7501,11 b' def tag(ui, repo, name1, *names, **opts)'
                 )
         node = logcmdutil.revsingle(repo, rev_).node()
 
+        # don't allow tagging the null rev or the working directory
         if node is None:
             raise error.InputError(_(b"cannot tag working directory"))
+        elif not opts.get(b'remove') and node == nullid:
+            raise error.InputError(_(b"cannot tag null revision"))
 
         if not message:
             # we don't translate commit messages
@@ -7522,13 +7526,6 b' def tag(ui, repo, name1, *names, **opts)'
             editform=editform, **pycompat.strkwargs(opts)
         )
 
-        # don't allow tagging the null rev
-        if (
-            not opts.get(b'remove')
-            and logcmdutil.revsingle(repo, rev_).rev() == nullrev
-        ):
-            raise error.InputError(_(b"cannot tag null revision"))
-
         tagsmod.tag(
             repo,
             names,