Squash away merge commits!

[Daniel Smith]

Recently our publisher robot broke and it turns out the reason is non-empty merge commits.

Non-empty merge commits happen if you do a `git merge` and have to resolve conflicts.

If you sync to head with `git fetch` / `git rebase`, you will end up fixing your commits instead of making merge commits. This makes for a much more readable commit history, and is in line with the spirit of our current commit guidelines.

...but I realized that our developer docs weren't clear on this point, so I've proposed https://github.com/kubernetes/community/pull/966 to make them clear. Let's argue there if this isn't non-controversial.

We might add yet another verification script ( :( :( :( ) to detect & prevent these, but in the meantime, reviewers: please keep your eye out for merge commits and request that they be squashed away. Thanks!