I think this failure repros in master with the latest updates from libgit2.

Hello, please rebase or merge against master, there are conflicts since we moved to libgit2 v0.21.0

Otherwise it looks good, just the second patch does not respect the 4 spaces indentation coding style.

Rebased and fixed the style. I noticed in the docs it mentions that all APIs that take an Oid should also accept a hex string. What's the right way to make this work for Repository_head__set__?

Regarding git_merge_trees, I don't know much about that side of the code, maybe @carlosmn wants to comment.

git_merge_trees is a fairly low-level operation, which doesn't necessarily belong anywhere in particular. Since its first input parameter is the repository, we could hang if off there as Repository.merge_trees(), but it's not really a repository operation, so it would fit just as well as pygit2.merge_trees() which takes in a Repository as its first argument.

If it takes a repository, then I vote for Repository.merge_trees()

I actually ended up going with TreeBuilder and manually merging things, because what I needed was more like the "refuse to overwrite entries that already existed in the original index file" behavior of git read-tree --prefix. I couldn't see a way to specify different merge algorithms for merge_trees. Any chance I missed something?