Currently most (all? haven't checked) discovery repositories are set to the default submit type in gerrit, specifically Merge if necessary. Basically this means gerrit will try to fast forward, and if that fails it will create a merge commit. Alternate submit types, used in a variety of operations/* and integration/* repos, are Rebase if necessary and Fast Forward Only. Both of these options eliminate merge commits and put git log in the order that commits are released, the primary difference is Fast Forward Only requires the user to rebase, while Rebase if necesssary lets gerrit try.
The motivation here was that I, being quite silly, misread a git log and attributed a bug to the position in the log that the commit changing things shows up, instead of at the merge commit when that code was applied. Putting the log in release order will help simplify tracking down bugs in these logs. This will also reduce spam in the logs from merge commits, as they will no longer be necessary.
It's not clear yet if Rebase if necessary or Fast Forward Only is the preferred option. Rebase is the simplest, while fast forward is the most explicit. In the end for most of the discovery repos requiring fast forward is likely overkill.