clone.sh/update.sh: set fastimport.unpackLimit to 0

clone.sh/update.sh: set fastimport.unpackLimit to 0

Prior to Git v2.10.0 fast-import always produced packs and terrible
ones at that.

Starting with Git v2.10.0 it still produces horrible packs but will
unpack them by default if they have 100 or fewer objects in them.

We prefer the old behavior so we must set fastimport.unpackLimit to 0.

Note that the documentation says less than, but the code implements it
as less than or equal so we need to use 0 for fastimport.unpackLimit
whereas for the other three settings we need to use 1 because they are
properly implemented as < and because a setting of 0 means something
else for one of them.

Fortunately, setting fastimport.unpackLimit does correctly override
transfer.unpackLimit in the new code in contrast to the reverse in the
old code for the other three settings so we can successfully set the
fastimport limit to 0 while the others are set to 1.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>