Merge pull request libgit2#1743 from libgit2/ethomson/0.26.2 Release LibGit2Sharp v0.26.2 with a libgit2 update

Merge pull request libgit2#1700 from libgit2/users/ethomson/0_28_3 Update libgit2 to v0.28.3

Release 0.26 

Merge pull request libgit2#1661 from libgit2/ethomson/0_28pre1 Update libgit2 to v0.28.0-pre1

Merge pull request libgit2#1644 from libgit2/ethomson/0_25 Prepare for v0.25.4

Prepare for v0.25.3 release 

v0.25: update version.json 

Merge pull request libgit2#1577 from libgit2/ethomson/0_25_1 Update for LibGit2Sharp v0.25.1

Merge pull request libgit2#1556 from libgit2/ethomson/release_025 Release 0.25

Merge pull request libgit2#1541 from libgit2/ethomson/update_libgit2 Update libgit2 to pre-0.27