Update libgit2 to 91fa31f

https://github.com/libgit2/libgit2/compare/3f8d005...91fa31f
Release LibGit2Sharp v0.20.1
Merge remote-tracking branch 'upstream/master' into net35

Conflicts:
	Lib/NativeBinaries/amd64/git2-3f8d005.pdb
	Lib/NativeBinaries/amd64/git2-91fa31f.pdb
	Lib/NativeBinaries/x64/git2-3f8d005.dll
	Lib/NativeBinaries/x64/git2-3f8d005.pdb
	LibGit2Sharp/Properties/AssemblyInfo.cs
We use x64 not amd64

Need to sort out a neater way of updating this
Update nuspec (will try to merge versions soon)