Bump libgit2 to bd62dc6fdde123339844dc199a6c3b8ba0630afa
Make use of the new git_remote_fetch signature to clean up some code.
Merge pull request #428 from libgit2/arthur/remote-fetch-updates

Update `Remote#fetch` internals
Add `Repository#merge_bases`.

This returns an array containing all merge bases between one or
multiple commits.