diff -r 539cbe0f8fa3 -r df3cf9422e1b mercurial/changegroup.py
--- a/mercurial/changegroup.py	Wed May 10 16:17:58 2017 -0700
+++ b/mercurial/changegroup.py	Mon May 15 09:35:27 2017 -0700
@@ -494,9 +494,18 @@
 class cg1packer(object):
     deltaheader = _CHANGEGROUPV1_DELTA_HEADER
     version = '01'
-    def __init__(self, repo):
+    def __init__(self, repo, bundlecaps=None):
         """Given a source repo, construct a bundler.
+
+        bundlecaps is optional and can be used to specify the set of
+        capabilities which can be used to build the bundle. While bundlecaps is
+        unused in core Mercurial, extensions rely on this feature to communicate
+        capabilities to customize the changegroup packer.
         """
+        # Set of capabilities we can use to build the bundle.
+        if bundlecaps is None:
+            bundlecaps = set()
+        self._bundlecaps = bundlecaps
         # experimental config: bundle.reorder
         reorder = repo.ui.config('bundle', 'reorder', 'auto')
         if reorder == 'auto':
@@ -800,8 +809,8 @@
     version = '02'
     deltaheader = _CHANGEGROUPV2_DELTA_HEADER
 
-    def __init__(self, repo):
-        super(cg2packer, self).__init__(repo)
+    def __init__(self, repo, bundlecaps=None):
+        super(cg2packer, self).__init__(repo, bundlecaps)
         if self._reorder is None:
             # Since generaldelta is directly supported by cg2, reordering
             # generally doesn't help, so we disable it by default (treating
@@ -895,9 +904,9 @@
     assert versions
     return min(versions)
 
-def getbundler(version, repo):
+def getbundler(version, repo, bundlecaps=None):
     assert version in supportedoutgoingversions(repo)
-    return _packermap[version][0](repo)
+    return _packermap[version][0](repo, bundlecaps)
 
 def getunbundler(version, fh, alg, extras=None):
     return _packermap[version][1](fh, alg, extras=extras)
@@ -948,24 +957,26 @@
     bundler = getbundler(version, repo)
     return getsubset(repo, outgoing, bundler, source)
 
-def getlocalchangegroupraw(repo, source, outgoing, version='01'):
+def getlocalchangegroupraw(repo, source, outgoing, bundlecaps=None,
+                           version='01'):
     """Like getbundle, but taking a discovery.outgoing as an argument.
 
     This is only implemented for local repos and reuses potentially
     precomputed sets in outgoing. Returns a raw changegroup generator."""
     if not outgoing.missing:
         return None
-    bundler = getbundler(version, repo)
+    bundler = getbundler(version, repo, bundlecaps)
     return getsubsetraw(repo, outgoing, bundler, source)
 
-def getchangegroup(repo, source, outgoing, version='01'):
+def getchangegroup(repo, source, outgoing, bundlecaps=None,
+                   version='01'):
     """Like getbundle, but taking a discovery.outgoing as an argument.
 
     This is only implemented for local repos and reuses potentially
     precomputed sets in outgoing."""
     if not outgoing.missing:
         return None
-    bundler = getbundler(version, repo)
+    bundler = getbundler(version, repo, bundlecaps)
     return getsubset(repo, outgoing, bundler, source)
 
 def getlocalchangegroup(repo, *args, **kwargs):