Recommend these .gitconfig settings for git integrity.

[Karl Fogel]

I've just added these settings to my ~/.gitconfig.  Based on some recent 
reports, it might be a good idea for all of us to do the same:

  [transfer]
          fsckObjects = true
  [fetch]
          fsckObjects = true
  [receive]
          fsckObjects = true

Summary:

Although git communicates object ID by content-addressable hashes (thus in 
theory ensuring integrity), git apparently doesn't always bother to actually 
*check* the hashes, e.g., when receiving objects from remote repositories.  
Enabling the above settings causes git to notice if someone ships you a bogus 
object, which seems like, er, a win.  You might be worried about a slowdown in 
some git operations, since content would now be checked against a hash, but 
according to those who've enabled the settings there's no noticeable slowdown 
in practice.

So, based on the discussion in the thread below, there are good reasons for 
everyone to enable these settings, and no reason not to.  (I was kind of 
surprised they weren't turned on by default in git, actually.)

See this post & thread for more details:

  From: Eric Myhre
  Subject: git integrity
  To: binary-transparency (Google Group)
  Message-ID: <address@hidden>
  Date: Fri, 29 Jan 2016 11:15:39 -0800

  https://groups.google.com/forum/#!topic/binary-transparency/f-BI4o8HZW0

See also these bug tickets mentioned by DKG in the thread:

  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=813157
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=743227

If we have consensus here, I could add this recommendation to the 'CONTRIBUTE' 
file in the Emacs tree; I'll wait to see what the followup is before doing 
that, however.

Best regards,
-Karl