git checkout master git merge PreviousRelease --strategy ours
commonly to preserve a relationship between branches without actually taking content from the "PreviousRelease" branch. This is useful, for example, in case of commits which simply bump the version of the release.
Those merge commits don't have an associated Differential revision because they have no content, but Diffusion history browser lists them with a revision that is the same as the revision of the commit before. Not a big deal, but a little bit confusing, since if you click on the commit to see the details, the revision will not be listed.
Notice how in the image above D703 is listed twice, even though it only applies to the first commit, not the merge commit.