[PATCH 0 of 1] bookmarks: fix for recursive loop when bookmarks file has invalid SHA

[Paul Molodowitch]

make _bookmarks use changelog.lookup, instead of localrepo.lookup, to avoid
recursive loop, which occurs if there is an invalid sha in the bookmarks file:
ie, _bookmarks calls localrepo.lookup, which will call localrepo.tags(), which
calls bookmarks._findtags, which calls _bookmarks, etc...