builtin/repack.c: assign pack split later

builtin/repack.c: assign pack split later

To determine the where to place the split when repacking with the
'--geometric' option, split_pack_geometry() assigns the "split" variable
and then decrements it in a loop.

It would be equivalent (and more readable) to assign the split to the
loop position after exiting the loop, so do that instead.

Suggested-by: Junio C Hamano <redacted>
Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>