Create objects using go-git

[nyancrimew]

Instead of calling out to the git command twice to hash the missing files and then check them out, do the same thing natively using go-git.

I am not sure if this is possible, but it probably should be.

[nyancrimew]

At the very least i should only call each command once, instead of per file

[nyancrimew]

Objects are now created fully in go and file metadata is updated from the index directly. This is an immense speed up to the object creation compared to doing it via the git commandline.