builtin rebase: support `--autosquash`

builtin rebase: support `--autosquash`

This commit adds support for the `--autosquash` option which is used to
automatically squash the commits marked as `squash` or `fixup` in their
messages. This is converted following `git-legacy-rebase.sh` closely.

This option can also be configured via the Git config setting
rebase.autosquash. To support this, we also add a custom
rebase_config() function in this commit that will be used instead (and
falls back to) 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>