Update to libgit2@0.21.1.
Bump the version number for a maintenance release.
Add a changelog file.
libgit2: Update to 0.21.2.
Release 0.21.2.
Bump the rugged version to v0.22.0.dev.
Merge branch 'maint/v0.21' of github.com:libgit2/rugged
Remove changelog entries for the 0.21.1b* releases.