Merge two commits, producing a git_index that reflects the result of the merge. The index may be written as-is to the working directory or checked out. If the index is to be converted to a tree, the caller should resolve any conflicts that arose as part of the merge.

The merge performed uses the first common ancestor, unlike the git-merge-recursive strategy, which may produce an artificial common ancestor tree when there are multiple ancestors.

The returned index must be freed explicitly with git_index_free.