Release process: don't assume the remove tracking branch is up-to-date.

wiki/src/contribute/release_process.mdwn

@@ -90,7 +90,7 @@ If we are at freeze time for a major release:
 
 1. Merge the `master` Git branch into `devel`:
 
-        git checkout devel && git merge --no-ff origin/master
+        git checkout devel && git fetch origin && git merge --no-ff origin/master
 
 2. [[Merge each APT overlay suite|APT_repository/custom#workflow-merge-overlays]]
    listed in the `devel` branch's `config/APT_overlays.d/` into the `devel`