documentation: replace Discussion section by link to user-manual chapter

documentation: replace Discussion section by link to user-manual chapter

The "Discussion" section has a lot of useful information, but is a
little wordy, especially for an already-long man page, and is designed
for an audience more of potential git hackers than users, which probably
doesn't make as much sense as git matures.  Also, I (perhaps foolishly)
forked a version in the user manual, which has been significantly
rewritten in an attempt to address some of the above problems.

So, remove this section and replace it by a (very terse) summary of the
original material--my attempt at the World's Shortest Git Overview--and
a reference to the appropriate chapter of the user manual.  It's
unfortunate to remove something that's been in this place for a long
time, as some people may still depend on finding it there.  But I think
we'll want to do this some day anyway.

Cc: Andreas Ericsson <ae@op5.se>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>