plumbing/packfile: add index generation to decoder
plumbing/packfile: add index generation to decoder

Signed-off-by: Javi Fontan <bd6f2f0212e303b54f36d84bb19ae60fa8e400a6@gmail.com>

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