plumbing/pacfile: tidy up objectInfo struct
plumbing/pacfile: tidy up objectInfo struct

* a new hasher is created when needed
* delete unused fields
* base content is no longer kept in memory

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

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