Because the 2.0 code was branched into RELENG_2_0 and mainline was opened back up for major changes, it was time to merge the IPv6 code into mainline to start paving the road for 2.1. (And there was much rejoicing, yay!)
As of this morning, the merge is complete, and the IPv6 code is in the pfsense code repository on the master branch (mainline).
2.0 snapshots are not built from mainline, but from RELENG_2_0 so a gitsync is still needed to get IPv6 onto current snapshots. You can use the standard pfsense repo with the "master" branch for gitsync at this stage to have the same net effect as following the previous IPv6 repository.
Once 2.0 is released the snapshots will start building from mainline again and IPv6 will not require any special settings of syncing at that point.