Arbitrary file overwrite via read-tree injection

Argument injection in GitPython before 3.1.58 allows attackers to overwrite arbitrary writable files via crafted treeish input. IndexFile.from_tree, IndexFile.reset, and IndexFile.merge_tree append caller-influenced treeish values to git read-tree without an unsafe-option check or a -- separator, so an injected --index-output option replaces the internal temporary index path. Reachability depends on an application passing untrusted treeish strings into those methods; successful exploitation clobbers the chosen writable path with a Git index blob, destroying existing content without yielding code execution.