copies: prepare changelog for more copies storage mode

We want to store copies information in sidedata, the logic will be quite similar
to the one used for extra, but sightly different (not in extra, no hash impact).

We start with small refactoring to make our work easier and the coming changesets
cleaners.

Differential Revision: https://phab.mercurial-scm.org/D6936

mercurial/changelog.py

@@ -391,6 +391,7 b' class changelog(revlog.revlog):'
         self._delaybuf = None
         self._divert = False
         self.filteredrevs = frozenset()
+        self._copiesstorage = opener.options.get('copies-storage')
 
     def tiprev(self):
         for i in pycompat.xrange(len(self) -1, -2, -1):
@@ -633,13 +634,14 b' class changelog(revlog.revlog):'
             elif branch in (".", "null", "tip"):
                 raise error.StorageError(_('the name \'%s\' is reserved')
                                          % branch)
-        extrasentries = p1copies, p2copies, filesadded, filesremoved
-        if extra is None and any(x is not None for x in extrasentries):
-            extra = {}
         sortedfiles = sorted(files)
         if extra is not None:
             for name in ('p1copies', 'p2copies', 'filesadded', 'filesremoved'):
                 extra.pop(name, None)
+        if self._copiesstorage == 'extra':
+            extrasentries = p1copies, p2copies, filesadded, filesremoved
+            if extra is None and any(x is not None for x in extrasentries):
+                extra = {}
         if p1copies is not None:
             extra['p1copies'] = encodecopies(sortedfiles, p1copies)
         if p2copies is not None:

mercurial/localrepo.py

@@ -751,6 +751,11 b' def resolvestorevfsoptions(ui, requireme'
     else: # explicitly mark repo as using revlogv0
         options['revlogv0'] = True
 
+    writecopiesto = ui.config('experimental', 'copies.write-to')
+    copiesextramode = ('changeset-only', 'compatibility')
+    if (writecopiesto in copiesextramode):
+        options['copies-storage'] = 'extra'
+
     return options
 
 def resolverevlogstorevfsoptions(ui, requirements, features):