cache-tree: protect against "git prune".

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.

Signed-off-by: Junio C Hamano <junkio@cox.net>

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