Change order of list items in return value from git2r_odb_blobs

* src/git2r_odb.c/h (git2r_odb_blobs): Reorder list items so that blob
  information and commit information are grouped together:
  - sha: the sha of the blob
  - path: the path to the blob from the tree and sub-trees
  - name: the name of the blob from the tree that contains the blob
  - len: the length of the blob
  - commit: the sha of the commit
  - author: the author of the commit
  - when: the timestamp of the author signature in the commit

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
List blobs in the object database

* R/odb.r (odb_blobs): List all blobs reachable from the commits in
  the object database. For each commit, list blob's in the commit tree
  and sub-tree's. Return a data.frame with the following columns:
  - sha: The sha of the blob.
  - path: The path to the blob from the tree and sub-trees.
  - name: The name of the blob from the tree that contains the blob.
  - len: The length of the blob.
  - commit: The sha of the commit.
  - author: The author of the commit.
  - when: The timestamp of the author signature in the commit.

* Note: A blob sha can have several entries

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>