Merge two trees, producing a git_index that reflects the result of the merge.
git_index
The returned index must be freed explicitly with git_index_free.
git_index_free
pointer to store the index result in
repository that contains the given trees
the common ancestor between the trees (or null if none)
the tree that reflects the destination tree
the tree to merge in to our_tree
our_tree
the merge tree options (or null for defaults)
zero on success, -1 on failure.