builtin/repo: add keyvalue and nul format for structure stats

All repository structure stats are outputted in a human-friendly table
form. This format is not suitable for machine parsing. Add a --format
option that supports three output modes: `table`, `keyvalue`, and `nul`.
The `table` mode is the default format and prints the same table output
as before.

With the `keyvalue` mode, each line of output contains a key-value pair
of a repository stat. The '=' character is used to delimit between keys
and values. The `nul` mode is similar to `keyvalue`, but key-values are
delimited by a NUL character instead of a newline. Also, instead of a
'=' character to delimit between keys and values, a newline character is
used. This allows stat values to support special characters without
having to cquote them. These two new modes provides output that is more
machine-friendly.

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

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