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>

Documentation/technical/commit-graph.adoc

@@ -39,6 +39,7 @@ A consumer may load the following info for a commit from the graph:
 Values 1-4 satisfy the requirements of parse_commit_gently().
 
 There are two definitions of generation number:
+
 1. Corrected committer dates (generation number v2)
 2. Topological levels (generation number v1)
 
@@ -158,7 +159,8 @@ number of commits in the full history. By creating a "chain" of commit-graphs,
 we enable fast writes of new commit data without rewriting the entire commit
 history -- at least, most of the time.
 
-## File Layout
+File Layout
+~~~~~~~~~~~
 
 A commit-graph chain uses multiple files, and we use a fixed naming convention
 to organize these files. Each commit-graph file has a name
@@ -170,11 +172,11 @@ hashes for the files in order from "lowest" to "highest".
 
 For example, if the `commit-graph-chain` file contains the lines
 
-```
+----
 	{hash0}
 	{hash1}
 	{hash2}
-```
+----
 
 then the commit-graph chain looks like the following diagram:
 
@@ -213,7 +215,8 @@ specifying the hashes of all files in the lower layers. In the above example,
 `graph-{hash1}.graph` contains `{hash0}` while `graph-{hash2}.graph` contains
 `{hash0}` and `{hash1}`.
 
-## Merging commit-graph files
+Merging commit-graph files
+~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 If we only added a new commit-graph file on every write, we would run into a
 linear search problem through many commit-graph files.  Instead, we use a merge
@@ -225,6 +228,7 @@ is determined by the merge strategy that the files should collapse to
 the commits in `graph-{hash1}` should be combined into a new `graph-{hash3}`
 file.
 
+....
 			    +---------------------+
 			    |                     |
 			    |    (new commits)    |
@@ -250,6 +254,7 @@ file.
  |                       |
  |                       |
  +-----------------------+
+....
 
 During this process, the commits to write are combined, sorted and we write the
 contents to a temporary file, all while holding a `commit-graph-chain.lock`
@@ -257,14 +262,15 @@ lock-file.  When the file is flushed, we rename it to `graph-{hash3}`
 according to the computed `{hash3}`. Finally, we write the new chain data to
 `commit-graph-chain.lock`:
 
-```
+----
 	{hash3}
 	{hash0}
-```
+----
 
 We then close the lock-file.
 
-## Merge Strategy
+Merge Strategy
+~~~~~~~~~~~~~~
 
 When writing a set of commits that do not exist in the commit-graph stack of
 height N, we default to creating a new file at level N + 1. We then decide to
@@ -289,7 +295,8 @@ The merge strategy values (2 for the size multiple, 64,000 for the maximum
 number of commits) could be extracted into config settings for full
 flexibility.
 
-## Handling Mixed Generation Number Chains
+Handling Mixed Generation Number Chains
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 With the introduction of generation number v2 and generation data chunk, the
 following scenario is possible:
@@ -318,7 +325,8 @@ have corrected commit dates when written by compatible versions of Git. Thus,
 rewriting split commit-graph as a single file (`--split=replace`) creates a
 single layer with corrected commit dates.
 
-## Deleting graph-{hash} files
+Deleting graph-\{hash\} files
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 After a new tip file is written, some `graph-{hash}` files may no longer
 be part of a chain. It is important to remove these files from disk, eventually.
@@ -333,7 +341,8 @@ files whose modified times are older than a given expiry window. This window
 defaults to zero, but can be changed using command-line arguments or a config
 setting.
 
-## Chains across multiple object directories
+Chains across multiple object directories
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 In a repo with alternates, we look for the `commit-graph-chain` file starting
 in the local object directory and then in each alternate. The first file that