git: restore basic functionality after b3040b6739ce

We don't yet have a formal interface for the changelog, but it's
mostly specified. Sadly, b3040b6739ce added a semi-private pseudo-enum
that we need to cope with, so it's probably high time that someone
(me?) attempts to define that interface to prevent future backsliding.

Differential Revision: https://phab.mercurial-scm.org/D8992

hgext/git/gitlog.py

@@ -96,6 +96,10 b' class baselogindex(object):'
 
 # TODO: an interface for the changelog type?
 class changelog(baselog):
+    # TODO: this appears to be an enumerated type, and should probably
+    # be part of the public changelog interface
+    _copiesstorage = b'extra'
+
     def __contains__(self, rev):
         try:
             self.node(rev)