Apply transfer.fsckobjects true to our default Git config

Originally created by @sajolida on #11107 (Redmine)

See:

• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=813157
• https://mailman.boum.org/pipermail/tails-dev/2016-February/010186.html

Proposed but untested patch:

--- a/config/chroot_local-includes/etc/gitconfig
+++ b/config/chroot_local-includes/etc/gitconfig
@@ -1,2 +1,4 @@
 [color]
        ui = auto
+[transfer]
+       fsckObjects = true