plumbing/packfile: do not compute sha1 for already undeltified objects
plumbing/packfile: do not compute sha1 for already undeltified objects

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

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