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: J. Bruce Fields <email@example.com>