doc: commit-graph.adoc: fix up some formatting

doc: commit-graph.adoc: fix up some formatting

The formatting markup syntax used in this document (markdown?) is not
interpreted correctly by asciidoc or asciidoctor. The main problem is
the use of a '## ' prefix markup for some sub-headings, along with the
use of '```' code markup and some missing literal blocks.

In order to improve the (html) document formatting:

  - replace the '## ' prefix sub-title syntax with the '~~' underlining
    syntax for the relevant sub-headings.
  - replace the '```' code markup, which causes asciidoc(tor) to simply
    remove the marked up text, with a literal block '----' markup.
  - the second ascii diagram, in the 'Merging commit-graph files'
    section, is not rendered correctly by asciidoctor (asciidoc is fine)
    so enclose it in a '....' block.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>