We have two similar functions, `git_treebuilder_insert` and `append_entry` which
are used in different codepaths as part of creating a new tree. The former
learnt to check for object existence under strict object creation, but the
latter did not.

This allowed the creation of a tree from an unowned index to bypass some of the
checks and create a tree pointing to a nonexistent object.

Extract a single function which performs these checks and call it from both
codepaths. In `append_entry` we still do not validate when asked not to, as this
is data which is already in the tree and we want to allow users to deal with
repositories which already have some invalid data.