bookmarks: create undo.bookmarks using repo.opener instead of util.copyfile

This more closely matches how the other undo files are created, and we
don't care about settings permissions or times on the file, which can
fail if the user running hg doesn't own the file.

hgext/bookmarks.py

@@ -44,8 +44,14 b' def write(repo):'
     can be copied back on rollback.
     '''
     refs = repo._bookmarks
-    if os.path.exists(repo.join('bookmarks')):
-        util.copyfile(repo.join('bookmarks'), repo.join('undo.bookmarks'))
+
+    try:
+        bms = repo.opener('bookmarks').read()
+    except IOError:
+        bms = None
+    if bms is not None:
+        repo.opener('undo.bookmarks', 'w').write(bms)
+
     if repo._bookmarkcurrent not in refs:
         setcurrent(repo, None)
     wlock = repo.wlock()