cache-tree: do not try to use an invalidated subtree info to build a tree

We punt from repairing the cache-tree during a branch switching if
it involves having to create a new tree object that does not yet
exist in the object store.  "mkdir dir && >dir/file && git add dir"
followed by "git checkout" is one example, when a tree that records
the state of such "dir/" is not in the object store.

However, after discovering that we do not have a tree object that
records the state of "dir/", the caller failed to remember the fact
that it noticed the cache-tree entry it received for "dir/" is
invalidated, it already knows it should not be populating the level
that has "dir/" as its immediate subdirectory, and it is not an
error at all for the sublevel cache-tree entry gave it a bogus
object name it shouldn't even look at.

This led the caller to detect and report a non-existent error.  The
end result was the same and we avoided stuffing a non-existent tree
to the cache-tree, but we shouldn't have issued an alarming error
message to the user.

Signed-off-by: Junio C Hamano <gitster@pobox.com>