changegroup: don't fail on empty changegroup (API)

I don't know why applying an empty changegroup should be an error. It
seems harmless. I suspect the check was there to find code that
creates empty changegroups just because that would be wasteful. Let's
use develwarn() for that instead, so we catch any such cases that run
with our test runner, but we still allow others to generate empty
changegroups if they want to.

We have run into this check at Google once or twice and had to work
around it, but I'm changing this not so much because of that, but
because it seems like it shouldn't be an error.

I also changed the message slightly to be more modern ("changelog
group" -> "changegroup") and more generic ("received" -> "applied").

mercurial/changegroup.py

@@ -298,7 +298,8 b' class cg1unpacker(object):'
             efiles = len(efiles)
 
             if not cgnodes:
-                raise error.Abort(_("received changelog group is empty"))
+                repo.ui.develwarn('applied empty changegroup',
+                                  config='empty-changegroup')
             clend = len(cl)
             changesets = clend - clstart
             repo.ui.progress(_('changesets'), None)