changegroup: improve the devel-warn to specify changelog was empty

Right now, the develwarn says "applied empty changegroup" which is not correct
because we can send a changegroup without changelog with just manifest and
filelogs and it will still say the same.

Let's fix this to say that we are applying empty changelog from changegroup. In
future patches I am will be adding functionality to send a changegroup from the
server without an empty changelog.

Differential Revision: https://phab.mercurial-scm.org/D4636

mercurial/changegroup.py

@@ -313,7 +313,7 b' class cg1unpacker(object):'
             efiles = len(efiles)
 
             if not cgnodes:
-                repo.ui.develwarn('applied empty changegroup',
+                repo.ui.develwarn('applied empty changelog from changegroup',
                                   config='warn-empty-changegroup')
             clend = len(cl)
             changesets = clend - clstart