Documentation: not learning core git commands.

The initial section of tutorial was too heavy on internal
workings for the first-time readers, so rewrite the introductory
section of git(7) to start with "not learning core git commands"
section and refer them to README to grasp the basic concepts,
then Everyday to give overview with task/role oriented examples
for minimum set of commands, and finally the tutorial.

Also add to existing note in the tutorial that many too
technical descriptions can be skipped by a casual reader.

I initially started to review the tutorial, with the objective
of ripping out the detailed technical information altogether,
but I found that the level of details in the initial couple of
sections that talk about refs and the object database in a
hands-on fashion was about rigth, and left all of them there.  I
feel that reading about fsck-index and repack is too abstract
without being aware of these directories and files.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git.txt

@@ -33,34 +33,41 @@ OPTIONS
 	environment variable. If no path is given 'git' will print
 	the current setting and then exit.
 
-CORE GIT COMMANDS
------------------
-Before reading this cover to cover, you may want to take a look
-at the link:tutorial.html[tutorial] document.  If you are
-migrating from CVS, link:cvs-migration.html[cvs migration]
-document may be helpful after you finish the tutorial.
 
-The <<Discussion>> section below contains much useful definition
-and clarification info - read that first.  After that, if you
-are interested in using git to manage (version control)
+NOT LEARNING CORE GIT COMMANDS
+------------------------------
+
+This manual is intended to give complete background information
+and internal workings of git, which may be too much for most
+people.  The <<Discussion>> section below contains much useful
+definition and clarification - read that first.
+
+If you are interested in using git to manage (version control)
 projects, use link:everyday.html[Everyday GIT] as a guide to the
 minimum set of commands you need to know for day-to-day work.
+Most likely, that will get you started, and you can go a long
+way without knowing the low level details too much.
+
+The link:tutorial.html[tutorial] document covers how things
+internally work.
+
+If you are migrating from CVS, link:cvs-migration.html[cvs
+migration] document may be helpful after you finish the
+tutorial.
 
 After you get the general feel from the tutorial and this
 overview page, you may want to take a look at the
 link:howto-index.html[howto] documents.
 
+
+CORE GIT COMMANDS
+-----------------
+
 If you are writing your own Porcelain, you need to be familiar
 with most of the low level commands --- I suggest starting from
 gitlink:git-update-index[1] and gitlink:git-read-tree[1].
 
 
-David Greaves <david@dgreaves.com>
-08/05/05
-
-Updated by Junio C Hamano <junkio@cox.net> on 2005-05-05 and
-further on 2005-12-07 to reflect recent changes.
-
 Commands Overview
 -----------------
 The git commands can helpfully be split into those that manipulate
@@ -582,14 +589,16 @@ include::../README[]
 
 Authors
 -------
-	git's founding father is Linus Torvalds <torvalds@osdl.org>.
-	The current git nurse is Junio C Hamano <junkio@cox.net>.
-	The git potty was written by Andres Ericsson <ae@op5.se>.
-	General upbringing is handled by the git-list <git@vger.kernel.org>.
+* git's founding father is Linus Torvalds <torvalds@osdl.org>.
+* The current git nurse is Junio C Hamano <junkio@cox.net>.
+* The git potty was written by Andres Ericsson <ae@op5.se>.
+* General upbringing is handled by the git-list <git@vger.kernel.org>.
 
 Documentation
 --------------
-Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
+The documentation for git suite was started by David Greaves
+<david@dgreaves.com>, and later enhanced greatly by the
+contributors on the git-list <git@vger.kernel.org>.
 
 GIT
 ---

Documentation/tutorial.txt

@@ -18,7 +18,14 @@ doing.
 The core git is often called "plumbing", with the prettier user
 interfaces on top of it called "porcelain". You may not want to use the
 plumbing directly very often, but it can be good to know what the
-plumbing does for when the porcelain isn't flushing... 
+plumbing does for when the porcelain isn't flushing.
+
+The material presented here often goes deep describing how things
+work internally.  If you are mostly interested in using git as a
+SCM, you can skip them during your first pass.
+
+[NOTE]
+And those "too deep" descriptions are often marked as Note.
 
 
 Creating a git repository
@@ -252,6 +259,17 @@ tree. That's very useful.
 A common shorthand for `git-diff-files -p` is to just write `git
 diff`, which will do the same thing.
 
+------------
+$ git diff
+diff --git a/hello b/hello
+index 557db03..263414f 100644
+--- a/hello
++++ b/hello
+@@ -1 +1,2 @@
+ Hello World
++It's a new day for git
+------------
+
 
 Committing git state
 --------------------