tag: clarify cryptic error message when tagging null revision

mercurial/commands.py

@@ -5601,7 +5601,7 b' def tag(ui, repo, name1, *names, **opts)'
         # don't allow tagging the null rev
         if (not opts.get('remove') and
             scmutil.revsingle(repo, rev_).rev() == nullrev):
-            raise util.Abort(_("null revision specified"))
+            raise util.Abort(_("cannot tag null revision"))
 
         repo.tag(names, r, message, opts.get('local'), opts.get('user'), date)
     finally:

tests/test-tag.t

@@ -264,11 +264,11 b' tagging on null rev'
 
   $ hg init empty
   $ hg tag -R empty nullrev
-  abort: null revision specified
+  abort: cannot tag null revision
   [255]
 
   $ hg tag -R empty -r 00000000000 -f nulltag
-  abort: null revision specified
+  abort: cannot tag null revision
   [255]
 
   $ cd ..