You don't convert it, you ask for the tree you want. The tree entry tells you its name, mode and id. If you want that tree, you look it up.
git_tree_lookup(&tree, repo, git_tree_entry_id(tree_entry))
Question
Same as title. How convert git_tree_entry to git_tree in libgit2?
if ((error = git_tree_entry_bypath(&source_tree_entry, root_tree, path)) < 0)
return GIT_ENOTFOUND;
// How convert source_tree_entry to tree here?
entries_count = git_tree_entrycount(tree);
La solution
You don't convert it, you ask for the tree you want. The tree entry tells you its name, mode and id. If you want that tree, you look it up.
git_tree_lookup(&tree, repo, git_tree_entry_id(tree_entry))