plumbing: object, add Commit.Verify method
plumbing: object, add Commit.Verify method

Commit.Verify() performs PGP verification of a signed commit given an
armored keyring.

mcuadros/go-git,mcuadros/go-git,go-git/go-git,src-d/go-git,go-git/go-git,src-d/go-git