diff -r 8aacac09e222 -r e40af07e518e mercurial/scmutil.py
--- a/mercurial/scmutil.py	Tue Nov 10 19:52:30 2015 +0800
+++ b/mercurial/scmutil.py	Mon Nov 02 16:52:34 2015 +0000
@@ -1170,3 +1170,11 @@
     subprocess."""
     return _locksub(repo, repo.currentwlock(), 'HG_WLOCK_LOCKER', cmd, *args,
                     **kwargs)
+
+def gdinitconfig(ui):
+    """helper function to know if a repo should be created as general delta
+
+    This currently depends on a single config option but this will get more
+    complicated soon."""
+    # experimental config: format.generaldelta
+    return ui.configbool('format', 'generaldelta', False)