When we remove all entries in a tree, we should remove that tree from
its parent rather than include the empty tree.

This gives us trees with subdirectories, which the new test needs.

Instead of going through the usual steps of reading a tree recursively
into an index, modifying it and writing it back out as a tree, introduce
a function to perform simple updates more efficiently.

`git_tree_create_updated` avoids reading trees which are not modified
and supports upsert and delete operations. It is not as versatile as
modifying the index, but it makes some common operations much more
efficient.

Instead of copying over the data into the individual entries, point to
the originals, which are already in a format we can use.

Submodules don't exist in the objectdb and the code is making us try to
look for a blob with its commit id, which is obviously not going to
work.

Skip the test if the user wants to insert a submodule.

Use legitimate (existing) object IDs in tests so that we have the
ability to turn on strict object validation when running tests.

When `GIT_OPT_ENABLE_STRICT_OBJECT_CREATION` is turned on, validate
the tree and parent ids given to treebuilder insertion.