Update libgit2 to 6720eef

https://github.com/libgit2/libgit2/compare/36a80fd...6720eef
Introduce new merge functionality, including Pull, merge options, and
merging branches.

This includes a refactoring of the merge logic to support the new
scenarios. New functionality includes:

- Deprecate Network.Fetchheads, Repository.MergeHeads as these should be
  internal only.

- Introduce ability to pull the configured upstream branch for the current
  branch

- Introduce ability to merge a branch into the current branch.

- Introduce options to control merge behavior. The current exposed options
  include whether to commit the merge commit and the allowed merge types.