builtin rebase: support the `verbose` and `diffstat` options

builtin rebase: support the `verbose` and `diffstat` options

This commit introduces support for the `-v` and `--stat` options of
rebase.

The --stat option can also be configured via the Git config setting
rebase.stat. To support this, we also add a custom rebase_config()
function in this commit that will be used instead of (and falls back to
calling) git_default_config().

Signed-off-by: Pratik Karki <predatoramigo@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>