Repo.reset_index is broken

[sattlerc]

Suppose I have a repository with a staged file:

mkdir test-repo
cd test-repo
git init
git commit --allow-empty --allow-empty-message -m ''
touch file
git add file

Then I reset the index using dulwich:

import dulwich.repo
r = dulwich.repo.Repo('test-repo')
r.reset_index()

However, the index is not reset. It still contains file.

The cause seems to be in build_index_from_tree. The documentation claims that the existing index is wiped. However, the function starts with:

index = Index(index_path)

This reads in the existing content of the index.