user-manual.txt: assign preface an id and a title

Two among the three warnings raised by "make git.info" are related to the fact
that the preface has not id in user-manual.txt.

    user-manual.texi:15: warning: empty menu entry name in `* : idm4.'
    user-manual.texi:141: warning: @unnumbered missing argument

This causes asciidoc creating an empty preface and an empty title tag in
user-manual.xml which turns to be an empty node in user-manual.texi and
git.info. Consequently, one can notice in user-manual.texi and git.info
a node named "idm4" in the menu and the navigation bar. In emacs, the
first entry of the menu in the git info page is even displayed as empty.

This fix will name "Introduction" the preface and assign it an id.
The result can be seen in the files: user-manual.{xml, texi, html, pdf}
and git.info.

For future reference, the diff between old and new user-manual.xml,
user-manual.texi, git.info, user-manual.html (converted through
html2markdown) and user-manual.pdf (converted through pdftotext) are
attached.

    --- before/user-manual.xml	2021-04-04 03:58:47.758008722 +0200
    +++ after/user-manual.xml	2021-04-04 03:56:40.520551163 +0200
    @@ -7,8 +7,8 @@
     <bookinfo>
         <title>Git User Manual</title>
     </bookinfo>
    -<preface>
    -<title></title>
    +<preface id="_introduction">
    +<title>Introduction</title>
     <simpara>Git is a fast distributed revision control system.</simpara>
     <simpara>This manual is designed to be readable by someone with basic UNIX
     command-line skills, but no previous knowledge of Git.</simpara>

    --- before/user-manual.texi	2021-04-04 03:58:47.490005652 +0200
    +++ after/user-manual.texi	2021-04-04 03:56:40.520551163 +0200
    @@ -7,12 +7,12 @@
     * Git: (git).           A fast distributed revision control system
     @end direntry

    -@node Top, idm4, , (dir)
    +@node Top, Introduction, , (dir)
     @documentlanguage en
     @top Git User Manual

     @menu
    -* : idm4.
    +* Introduction::
     * Repositories and Branches::
     * Exploring Git history::
     * Developing with Git::
    @@ -137,8 +137,8 @@
     @end detailmenu
     @end menu

    -@node idm4, Repositories and Branches, Top, Top
    -@unnumbered
    +@node Introduction, Repositories and Branches, Top, Top
    +@unnumbered Introduction

     Git is a fast distributed revision control system.

    @@ -178,7 +178,7 @@
     Finally, see @ref{Notes and todo list for this manual} for ways that you can help make this manual more
     complete.

    -@node Repositories and Branches, Exploring Git history, idm4, Top
    +@node Repositories and Branches, Exploring Git history, Introduction, Top
     @chapter Repositories and Branches

     @menu

    --- before/git.info	2021-04-04 03:58:46.557994966 +0200
    +++ after/git.info	2021-04-04 03:56:40.520551163 +0200
    @@ -7,14 +7,14 @@
     END-INFO-DIR-ENTRY

    -File: git.info,  Node: Top,  Next: idm4,  Up: (dir)
    +File: git.info,  Node: Top,  Next: Introduction,  Up: (dir)

     Git User Manual
     ***************

     * Menu:

    -* : idm4.
    +* Introduction::
     * Repositories and Branches::
     * Exploring Git history::
     * Developing with Git::
    @@ -137,7 +137,10 @@

    -File: git.info,  Node: idm4,  Next: Repositories and Branches,  Prev: Top,  Up: Top
    +File: git.info,  Node: Introduction,  Next: Repositories and Branches,  Prev: Top,  Up: Top
    +
    +Introduction
    +************

     Git is a fast distributed revision control system.

    @@ -174,7 +177,7 @@
     that you can help make this manual more complete.

    -File: git.info,  Node: Repositories and Branches,  Next: Exploring Git history,  Prev: idm4,  Up: Top
    +File: git.info,  Node: Repositories and Branches,  Next: Exploring Git history,  Prev: Introduction,  Up: Top

     1 Repositories and Branches
     ***************************
    @@ -5471,207 +5474,207 @@
    ...
     Tag Table:
     Node: Top212
    -Node: idm43164
    -Node: Repositories and Branches4465
    ...
    +Node: Introduction3179
    +Node: Repositories and Branches4515
    +Node: How to get a Git repository5128
    ...
    End Tag Table

    --- before/user-manual.html.md	2021-04-04 05:20:55.378695854 +0200
    +++ after/user-manual.html.md	2021-04-04 05:21:11.282850802 +0200
    @@ -4,6 +4,8 @@

      **Table of Contents**

    +Introduction
    +
     1\. Repositories and Branches

    @@ -278,7 +280,7 @@

     Todo list

    -#
    +# Introduction

     Git is a fast distributed revision control system.

    --- before/user-manual.pdf.txt	2021-04-04 05:28:20.367036836 +0200
    +++ after/user-manual.pdf.txt	2021-04-04 05:30:01.680026312 +0200
    @@ -487,6 +487,7 @@

     vii

    +Introduction
     Git is a fast distributed revision control system.
     This manual is designed to be readable by someone with basic UNIX command-line skills, but no previous knowledge of Git.
     Chapter 1 and Chapter 2 explain how to fetch and study a project using git—read these chapters to learn how to build and test a

Signed-off-by: Firmin Martin <firminmartin24@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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