split-index: don't write cache tree with null oid entries

In a96d3cc3f6 ("cache-tree: reject entries with null sha1", 2017-04-21)
we made sure that broken cache entries do not get propagated to new
trees.  Part of that was making sure not to re-use an existing cache
tree that includes a null oid.

It did so by dropping the cache tree in 'do_write_index()' if one of
the entries contains a null oid.  In split index mode however, there
are two invocations to 'do_write_index()', one for the shared index
and one for the split index.  The cache tree is only written once, to
the split index.

As we only loop through the elements that are effectively being
written by the current invocation, that may not include the entry with
a null oid in the split index (when it is already written to the
shared index), where we write the cache tree.  Therefore in split
index mode we may still end up writing the cache tree, even though
there is an entry with a null oid in the index.

Fix this by checking for null oids in prepare_to_write_split_index,
where we loop the entries of the shared index as well as the entries for
the split index.

This fixes t7009 with GIT_TEST_SPLIT_INDEX.  Also add a new test that's
more specifically showing the problem.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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