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

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 <bfields@citi.umich.edu>