[QA] Zuul +2 and fast forward repositories

[Antoine Musso]

Hello,

Paladox found a rather nice issue in Zuul which is that when you +2 two
patches on repository that is fast-forward only, only the first change
is merged in and the other is rejected.

We are using the fast-forward merge strategy on integration/config.git
repo since we want to manually review the JJB diff on every patch before
we land them.

I wrote an explanation on:
https://phabricator.wikimedia.org/T111426#1605970

I am pondering whether it should be documented on the wiki or maybe
amend upstream documentation.


-- 
Antoine "hashar" Musso