doc: convert git commit config to new format

Also prevent git-commit manpage to refer to itself in the config
description by using a variable.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/commit.txt

@@ -1,29 +1,34 @@
-commit.cleanup::
+ifdef::git-commit[]
+:see-git-commit:
+endif::git-commit[]
+ifndef::git-commit[]
+:see-git-commit: See linkgit:git-commit[1] for details.
+endif::git-commit[]
+`commit.cleanup`::
 	This setting overrides the default of the `--cleanup` option in
-	`git commit`. See linkgit:git-commit[1] for details. Changing the
-	default can be useful when you always want to keep lines that begin
+	`git commit`. {see-git-commit} Changing the default can be useful
+	when you always want to keep lines that begin
 	with the comment character `#` in your log message, in which case you
 	would do `git config commit.cleanup whitespace` (note that you will
 	have to remove the help lines that begin with `#` in the commit log
 	template yourself, if you do this).
 
-commit.gpgSign::
-
+`commit.gpgSign`::
 	A boolean to specify whether all commits should be GPG signed.
 	Use of this option when doing operations such as rebase can
 	result in a large number of commits being signed. It may be
 	convenient to use an agent to avoid typing your GPG passphrase
 	several times.
 
-commit.status::
+`commit.status`::
 	A boolean to enable/disable inclusion of status information in the
 	commit message template when using an editor to prepare the commit
-	message.  Defaults to true.
+	message.  Defaults to `true`.
 
-commit.template::
+`commit.template`::
 	Specify the pathname of a file to use as the template for
 	new commit messages.
 
-commit.verbose::
+`commit.verbose`::
 	A boolean or int to specify the level of verbosity with `git commit`.
-	See linkgit:git-commit[1].
+	{see-git-commit}