Documentation: revise top of git man page

Documentation: revise top of git man page

I'm afraid I'll be accused of trying to suck all the jokes and the
personality out of the git documentation.  I'm not!  Really!

That said, "man git" is one of the first things a new user is likely try,
and it seems a little cruel to start off with a somewhat obscure joke
about the architecture of git.

So instead I'm trying for a relatively straightforward description of what
git does, and what features distinguish it from other systems, together
with immediate links to introductory documentation.

I also did some minor reorganization in an attempt to clarify the
classification of commands.  And revised a bit for conciseness (as is
obvious from the diffstat--hopefully I didn't cut anything important).

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>