The git_merge_file_result to be filled in
The repository
The index entry for the ancestor file (stage level 1)
The index entry for our file (stage level 2)
The index entry for their file (stage level 3)
The merge file options or null
0 on success or error code
Merge two files as they exist in the index, using the given common ancestor as the baseline, producing a git_merge_file_result that reflects the merge result. The git_merge_file_result must be freed with git_merge_file_result_free.