Update libgit2 to f28e4c9

https://github.com/libgit2/libgit2/compare/36a80fd...f28e4c9
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.