Release process: automate.

wiki/src/contribute/release_process.mdwn

@@ -50,8 +50,7 @@ the scripts snippets found on this page:
   the `.packages` file) land.
 * `MASTER_CHECKOUT`: a checkout of the `master` branch of the main
   Tails Git repository.
-* `RELEASE_BRANCH`: the name of the branch of the main Tails Git
-  repository used to prepare the release (`stable` or `testing`).
+* `RELEASE_BRANCH=$(if [ "$MAJOR_RELEASE" = 1 ]; then echo -n testing; else echo -n stable; fi)`
 * `RELEASE_CHECKOUT`: a checkout of the branch of the main Tails Git
   repository used to prepare the release (`stable` or `testing`).
 * `TAILS_SIGNATURE_KEY=A490D0F4D311A4153E2BB7CADBB802B258ACD84F`