user-manual: adjust section levels in "git internals"

The descriptions of the various object types should all be a subsection
of the "Object Database" section.

I cribbed most of this chapter from the README (now core-intro.txt and
git(7)), because there's stuff in there people need to know and I was
too lazy to rewrite it.  The audience isn't quite right, though--the
chapter is a mixture of user- and developer- level documentation that
isn't as appropriate now as it was originally.

So, reserve this chapter for stuff users need to know, and move the
source code introduction into a new "git hacking" chapter where we'll
also move any hacker-only technical details.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -2787,7 +2787,7 @@ The object types in some more detail:
 
 [[blob-object]]
 Blob Object
------------
+~~~~~~~~~~~
 
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
@@ -2809,7 +2809,7 @@ is run, and its data can be accessed by gitlink:git-cat-file[1].
 
 [[tree-object]]
 Tree Object
------------
+~~~~~~~~~~~
 
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
@@ -2851,7 +2851,7 @@ Two trees can be compared with gitlink:git-diff-tree[1].
 
 [[commit-object]]
 Commit Object
--------------
+~~~~~~~~~~~~~
 
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
@@ -2878,7 +2878,7 @@ its data can be accessed by gitlink:git-cat-file[1].
 
 [[trust]]
 Trust
------
+~~~~~
 
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
@@ -2908,7 +2908,7 @@ To assist in this, git also provides the tag object...
 
 [[tag-object]]
 Tag Object
-----------
+~~~~~~~~~~
 
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
@@ -3474,6 +3474,13 @@ confusing and scary messages, but it won't actually do anything bad. In
 contrast, running "git prune" while somebody is actively changing the
 repository is a *BAD* idea).
 
+[[hacking-git]]
+Hacking git
+===========
+
+This chapter covers internal details of the git implementation which
+probably only git developers need to understand.
+
 [[birdview-on-the-source-code]]
 A birds-eye view of Git's source code
 -------------------------------------