Disable ignore_case when writing the index to a tree

This is a pretty bad bug that can result in corrupt trees being written by libgit2, if for example the index is sorted ignore_case and contains the following items:

Folder\a.txt
FOLDER\b.txt
Folder\c.txt

The logic in `write_tree` in `tree.c` will end up in this case adding "Folder" to the treebuilder twice.

The index *must* be sorted case-sensitively for tree-writing!