builtin/repo: humanise count values in structure output

builtin/repo: humanise count values in structure output

The table output format for the git-repo(1) structure subcommand is used
by default and intended to provide output to users in a human-friendly
manner. When the reference/object count values in a repository are
large, it becomes more cumbersome for users to read the values.

For larger values, update the table output format to instead produce
more human-friendly count values that are scaled down with the
appropriate unit prefix. Output for the keyvalue and nul formats remains
unchanged.

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