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

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.

--- a/hgext/bookmarks.py	Tue Dec 07 19:47:53 2010 +1100
+++ b/hgext/bookmarks.py	Tue Dec 07 20:03:04 2010 +1100
@@ -44,8 +44,14 @@
     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()