changegroup: mark _addchangegroupfiles as module-private

I'm trying to reason about the public surface area of this module now,
so it's worth tagging private things as such.

mercurial/changegroup.py

@@ -361,8 +361,8 b' class cg1unpacker(object):'
             repo.ui.status(_("adding file changes\n"))
             self.callback = None
             pr = prog(_('files'), efiles)
-            newrevs, newfiles = addchangegroupfiles(repo, self, revmap, trp, pr,
-                                                    needfiles)
+            newrevs, newfiles = _addchangegroupfiles(
+                repo, self, revmap, trp, pr, needfiles)
             revisions += newrevs
             files += newfiles
 
@@ -866,7 +866,7 b' def changegroup(repo, basenodes, source)'
     # to avoid a race we use changegroupsubset() (issue1320)
     return changegroupsubset(repo, basenodes, repo.heads(), source)
 
-def addchangegroupfiles(repo, source, revmap, trp, pr, needfiles):
+def _addchangegroupfiles(repo, source, revmap, trp, pr, needfiles):
     revisions = 0
     files = 0
     while True: