user-manual: rename "git internals" to "git concepts"

"git internals" sounds like something only git developers must know
about, but this stuff should be of wider interest.  Rename the chapter
and give it a slightly friendlier introduction.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -182,7 +182,7 @@ has that commit at all).  Since the object name is computed as a hash over the
 contents of the commit, you are guaranteed that the commit can never change
 without its name also changing.
 
-In fact, in <<git-internals>> we shall see that everything stored in git
+In fact, in <<git-concepts>> we shall see that everything stored in git
 history, including file data and directory contents, is stored in an object
 with a name that is a hash of its contents.
 
@@ -2708,12 +2708,16 @@ See gitlink:git-config[1] for more details on the configuration
 options mentioned above.
 
 
-[[git-internals]]
-Git internals
-=============
+[[git-concepts]]
+Git concepts
+============
 
-Git depends on two fundamental abstractions: the "object database", and
-the "current directory cache" aka "index".
+Git is built on a small number of simple but powerful ideas.  While it
+is possible to get things done without understanding them, you will find
+git much more intuitive if you do.
+
+We start with the most important, the  <<def_object_database,object
+database>> and the <<def_index,index>>.
 
 [[the-object-database]]
 The Object Database