builtin/repo: collect largest inflated objects

builtin/repo: collect largest inflated objects

The "structure" output for git-repo(1) shows the total inflated and disk
sizes of reachable objects in the repository, but doesn't show the size
of the largest individual objects. Since an individual object may be a
large contributor to the overall repository size, it is useful for users
to know the maximum size of individual objects.

While interating across objects, record the size and OID of the largest
objects encountered for each object type to provide as output. Note that
the default "table" output format only displays size information and not
the corresponding OID. In a subsequent commit, the table format is
updated to add table annotations that mention the OID.

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