docs/process: branching-checklist: Clarify steps to add the branch in osstest

The steps to add the new branch in osstest doesn't mention the change
should be committed.

Update the steps to clarify that. Take the opportunity to switch to
use $v instead hardcoded version.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Henry Wang <Henry.Wang@arm.com>

docs/process/branching-checklist.txt

@@ -49,7 +49,9 @@ ov=4.0
     OSSTEST_CONFIG=production-config-cambridge ./mg-branch-setup qemu-upstream-$v-testing bisect
 
 * add branch to osstest
-    ie add both eg qemu-upstream-4.2-testing and xen-4.2-testing to BRANCHES in cr-for-branches
+    Add both qemu-upstream-$v-testing and xen-$v-testing to BRANCHES in cr-for-branches
+    git add -p
+    git commit -m "cr-for-branches: Add Xen and QEMU $v branch"
 
 * add to patchbot
     on xenbits