devel-warn: add 'warn-' to 'devel.empty-changegroup' config

Rename 'devel.empty-changegroup' to 'devel.warn-empty- changegroup' in order
to clarify it controls a warning message.

No alias is installed since the previous configuration option was not
documented.

mercurial/changegroup.py

@@ -313,7 +313,7 b' class cg1unpacker(object):'
 
             if not cgnodes:
                 repo.ui.develwarn('applied empty changegroup',
-                                  config='empty-changegroup')
+                                  config='warn-empty-changegroup')
             clend = len(cl)
             changesets = clend - clstart
             repo.ui.progress(_('changesets'), None)

mercurial/configitems.py

@@ -220,7 +220,7 b" coreconfigitem('devel', 'deprec-warn',"
 coreconfigitem('devel', 'disableloaddefaultcerts',
     default=False,
 )
-coreconfigitem('devel', 'empty-changegroup',
+coreconfigitem('devel', 'warn-empty-changegroup',
     default=False,
 )
 coreconfigitem('devel', 'legacy.exchange',