builtin/repo: add keyvalue and nul format for stats

builtin/repo: add keyvalue and nul format for stats

All repository 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>