We reused the cache-tree data without verifying the tree object
still exists. Recompute in cache_tree_update() an otherwise
valid cache-tree entry when the tree object disappeared.
This is not usually a problem, but theoretically without this
fix things can break when the user does something like this:
- read-index from a side branch
- write-tree the result
- remove the side branch with "git branch -D"
- remove the unreachable objects with "git prune"
- write-tree what is in the index.