Please note that releases are made through BioConductor only, not through Github's release feature. At the time of each BioConductor release every ~6 months, a new branch is created from the master to keep each one in parallel so that they are still available whichever R+BioC version you are running. Once a BioC release is made, that version/branch is only allowed to get fixes/improvements that don't change the API.
The easiest way to find the releases history and their content is the NEWS file: https://github.com/broadinstitute/infercnv/blob/master/inst/NEWS
Visit our wiki for installation instructions and installation options:
https://github.com/broadinstitute/infercnv/wiki/Installing-infercnv