lfs: teach '{lfs_files}' to handle removed files

lfs: allow a pointer to be extracted from a context that removes the file This is needed to let 'set:lfs()' and '{lfs_files}' work normally on removed files. Yuya suggested returning a null pointer for removed files, instead of the pointer from the parent. The first attempt at this was to return None for a non LFS file, and a (pointer, ctx) tuple to hold the pointer and context (or parent pointer and context for a removed file). But this complicated the callers, even the ones that didn't care about removed files. Instead, let's use {} to represent a removed pointer. This has the added convenience of being a useful representation in the template language, and only affects the callers that care about removed files (and only slightly). Since pointers are explicitly serialized with a call to a member function, there is no danger of writing these to disk.