t5608: add regression test for >4GB object clone

t5608: add regression test for >4GB object clone

The shift overflow bug in index-pack and unpack-objects caused incorrect
object size calculation when the encoded size required more than 32 bits
of shift. This would result in corrupted or failed unpacking of objects
larger than 4GB.

Add a test that creates a pack file containing a 4GB+ blob using the
new 'test-tool synthesize pack --reachable-large' command, then clones
the repository to verify the fix works correctly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>