Update libgit2 to fd61f05

https://github.com/libgit2/libgit2/compare/36a80fd...fd61f05
Expand Merge API.

This includes refactoring of the merge logic to support different merge workflows, including pull.

Also adds a simple repository to perform merges in, simplying test logic.
Remove Network.FetchHeads from the public API.
Remove Repository.MergeHeads from public API.
Introduce PullOptions.
Write reflog entry as part of Fast-forward merge.
Update marshalling of git_merge_head_id
Update Fast-forward merge reflog message.
Teach Repository to merge a branch