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

Update LibGit2Sharp.nuspec

Merge pull request libgit2#1402 from libgit2/ethomson/0_23_1

Update natives to 1.0.164 (libgit2 a5cf255)