changegroup: avoid creating empty changegroup part

Previously this check happened in the changegroup code itself. Since its
refactor, this logic needs to move out to callers that care about it, such as
this one. Otherwise we get empty bundle devel-warnings in certain extensions.

Differential Revision: https://phab.mercurial-scm.org/D690

mercurial/exchange.py

@@ -1632,8 +1632,9 b' def _getbundlechangegrouppart(bundler, r'
                 raise ValueError(_('no common changegroup version'))
             version = max(cgversions)
         outgoing = _computeoutgoing(repo, heads, common)
-        cgstream = changegroup.makestream(repo, outgoing, version, source,
-                                          bundlecaps=bundlecaps)
+        if outgoing.missing:
+            cgstream = changegroup.makestream(repo, outgoing, version, source,
+                                              bundlecaps=bundlecaps)
 
     if cgstream:
         part = bundler.newpart('changegroup', data=cgstream)