update licenses from GPL to AGPL

This does not change the overall license of the git-annex program, which
was already AGPL due to a number of sources files being AGPL already.

Legally speaking, I'm adding a new license under which these files are
now available; I already released their current contents under the GPL
license. Now they're dual licensed GPL and AGPL. However, I intend
for all my future changes to these files to only be released under the
AGPL license, and I won't be tracking the dual licensing status, so I'm
simply changing the license statement to say it's AGPL.

(In some cases, others wrote parts of the code of a file and released it
under the GPL; but in all cases I have contributed a significant portion
of the code in each file and it's that code that is getting the AGPL
license; the GPL license of other contributors allows combining with
AGPL code.)

Git/Command.hs

@@ -2,7 +2,7 @@
  -
  - Copyright 2010-2013 Joey Hess <id@joeyh.name>
  -
- - Licensed under the GNU GPL version 3 or higher.
+ - Licensed under the GNU AGPL version 3 or higher.
  -}
 
 {-# LANGUAGE CPP #-}