Documentation: wrap config listings in "----"

Documentation: wrap config listings in "----"

The indented lines in these example config-file listings are indented
differently by AsciiDoc and Asciidoctor.

Fix this by marking the example config-files as code listings by
wrapping them in "----". Because this gives us some extra indentation,
we can remove the one that we have been carrying explicitly. That is,
drop the first tab of indentation on each line.

With AsciiDoc, this results in identical rendering before and after this
commit. Asciidoctor now renders this the same as AsciiDoc does.

git-config.txt pretty consistently uses twelve dashes rather than the
minimum four to spell "----". Let's stick to the file-local convention
there.

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