Documentation/gitpacking: make sample configs listing blocks

Documentation/gitpacking: make sample configs listing blocks

This document contains a few sample config snippets. At least with
Asciidoctor, the section headers are rendered *more* indented than the
variables that follow:

       [bitmapPseudoMerge "all"]
    pattern = "refs/"
    ...

To address this, wrap these listings in AsciiDoc listing blocks. Remove
the indentation from the section headings. This is similar to how we
handle such sample config elsewhere, e.g., in config.txt.

While we're here, fix the nearby "wiht" typo.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>