builtin/repo: add object counts in structure output

builtin/repo: add object counts in structure output

The amount of objects in a repository can provide insight regarding its
shape. To surface this information, use the path-walk API to count the
number of reachable objects in the repository by object type. All
regular references are used to determine the reachable set of objects.
The object counts are appended to the same table containing the
reference information.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>