fix overflow by passing ref ids in chunks of 2000
fix overflow by passing ref ids in chunks of 2000

This effectively reverts broken attempt at fixing #38, then applies a fix that calls git log in chunks of 2000. 200 worked for me, not sure whether we need a different chunk size for other OSes

unit test exists that would have failed for 4000 ref ids

wkentaro/vcstools,wkentaro/vcstools,jpgr87/vcstools