Summary of changes

• Bump Libgit2 to preview of v0.28.0
• Add signing support for commits and annotated tags
• Updated Signature.prototype.toString to optionally include timestamps
• [BREAKING] Converted Buf.prototype.set and Buf.prototype.grow from async to sync
• Added complete support for libgit2 types:
  • git_index_name_entry
  • git_index_reuc_entry
  • git_mailmap
• Exposed git_path_is_gitfile
• Exposed git_tag_create_frombuffer

Merged PRs into NodeGit

• adds support for gpg commit signing (fixes #1018) #1448
• Add updateRef parameter to Repository#createCommitWithSignature #1610
• Documentation fixes. #1611
• Add Commit#amendWithSignature #1616
• Bump libgit2 to a preview of v0.28 #1615
• Fix issues with Commit#amendWithSignature #1617
• Marked Repository.createBlobFromBuffer as async #1614
• Add functionality for creating Tags with signatures and extracting signatures from Tags #1618