Merge pull request libgit2#1396 from libgit2/cmn/changelog

Update CHANGES for the 0.23 release

Merge pull request libgit2#1268 from libgit2/cmn/update-v24

Update to libgit2 785d8c4

Release LibGit2Sharp v0.21.1

Release LibGit2Sharp v0.21

Release LibGit2Sharp v0.20.2

Release LibGit2Sharp v0.20.1

Release LibGit2Sharp v0.20

Release LibGit2Sharp v0.19

Release LibGit2Sharp v0.18.1

Release LibGit2Sharp v0.18.0