builtin/repo: add inflated object info to keyvalue structure output

builtin/repo: add inflated object info to keyvalue structure output

The structure subcommand for git-repo(1) outputs basic count information
for objects and references. Extend this output to also provide
information regarding total size of inflated objects by object type.

For now, object size by object type info is only added to the keyvalue
and nul output formats. In a subsequent commit, this info is also added
to the table format.

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