bundle: refactor changegroup prune to be its own function

Moving the prune function to be a non-nested function allows extensions to
control which revisions are allowed in the changegroup. For example, in my
shallow repo extension I want to prevent filelogs from being added to the
bundle.

This also allows an extension to use a filelog implementation that doesn't
have revlog.linkrev implemented.

mercurial/changegroup.py

@@ -296,6 +296,11 b' class bundle10(object):'
 
         yield self.close()
 
+    # filter any nodes that claim to be part of the known set
+    def prune(self, revlog, missing, commonrevs, source):
+        rr, rl = revlog.rev, revlog.linkrev
+        return [n for n in missing if rl(rr(n)) not in commonrevs]
+
     def generate(self, commonrevs, clnodes, fastpathlinkrev, source):
         '''yield a sequence of changegroup chunks (strings)'''
         repo = self._repo
@@ -311,11 +316,6 b' class bundle10(object):'
         fnodes = {} # needed file nodes
         changedfiles = set()
 
-        # filter any nodes that claim to be part of the known set
-        def prune(revlog, missing):
-            rr, rl = revlog.rev, revlog.linkrev
-            return [n for n in missing if rl(rr(n)) not in commonrevs]
-
         # Callback for the changelog, used to collect changed files and manifest
         # nodes.
         # Returns the linkrev node (identity in the changelog case).
@@ -347,7 +347,7 b' class bundle10(object):'
 
         for f in changedfiles:
             fnodes[f] = {}
-        mfnodes = prune(mf, mfs)
+        mfnodes = self.prune(mf, mfs, commonrevs, source)
         for chunk in self.group(mfnodes, mf, lookupmf, units=_('manifests'),
                                 reorder=reorder):
             yield chunk
@@ -377,7 +377,7 b' class bundle10(object):'
             def lookupfilelog(x):
                 return linkrevnodes[x]
 
-            filenodes = prune(filerevlog, linkrevnodes)
+            filenodes = self.prune(filerevlog, linkrevnodes, commonrevs, source)
             if filenodes:
                 progress(msgbundling, i + 1, item=fname, unit=msgfiles,
                          total=total)