storage/dotgit: search for incoming dir only once
storage/dotgit: search for incoming dir only once

Search for incoming object directory was done once each time objects
were accessed. This means a ReadDir of the objects path that is
expensive. Now incoming directory is searched the first time an object
is accessed and its name kept in DotGit to be reused.

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

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