Re-Z is part of the HyperZ family and is a check for Z pass/fail that does not update the Z buffer and allows doing multiple Z passes during one rendering. Re-Z support appears to extend back to the R600/R700 era.
With this Git commit, the RadeonSI Gallium3D driver supports making use of Re-Z for the newer AMD GCN GPUs. The commit by AMD's Marek Olšák simply explains, "This can increase perf for shaders that kill pixels (kill, alpha-test, alpha-to-coverage)."
This appears to be the first time the open-source AMD drivers (R600g or RadeonSI) have begun making use of Re-Z.
On a related note, the Mesa Git code has dropped support in RadeonSI, R600g, and R300g for kernels older than Linux 3.2. Support for DRM 2.12.0 or newer is now required, which will impact any users of RHEL 6, Debian 6, and other distributions with an older kernel while still wanting to try to use the latest Mesa code. Dropping this older DRM kernel support is being done in order to make it easier for the developers to change their winsys interface.