[blobs] blob symlinks are staged explicitely

Blobs (not git blobs) should not be put into git object storage. To be
sure it never happens they are added explicitely so bugs in the-vault
do not result in incidental git object store contamination

Signed-off-by: Denis Zalevskiy <denis.zalevskiy@jolla.com>
Merge pull request #4 from deztructor/master

[blobs] blob symlinks are staged explicitely