bookmarks: delegate writing to the repo just like reading

This makes it easier for alternate storage backends to not use flat
files for bookmarks storage.

mercurial/bookmarks.py

@@ -146,7 +146,7 b' def update(repo, parents, node):'
             marks[mark] = new.node()
             update = True
     if update:
-        write(repo)
+        repo._writebookmarks(marks)
 
 def listbookmarks(repo):
     # We may try to list bookmarks on a repo type that does not

mercurial/localrepo.py

@@ -167,6 +167,9 b' class localrepository(repo.repository):'
     def _bookmarkcurrent(self):
         return bookmarks.readcurrent(self)
 
+    def _writebookmarks(self, marks):
+      bookmarks.write(self)
+
     @filecache('00changelog.i', True)
     def changelog(self):
         c = changelog.changelog(self.sopener)