Ensure the stable branch is checked out before pulling in migrate.sh

The checkout would be a no-op if already on `stable` (git would print
`Already on 'stable'` but importantly would *not* return a failing exit
code). It would fail if the user has local changes, but if they have
local changes `git pull` would also fail, so there's no change there.

bin/omakub-sub/migrate.sh

@@ -1,5 +1,6 @@
 cd $OMAKUB_PATH
 last_updated_at=$(git log -1 --format=%cd --date=unix)
+git checkout stable
 git pull
 
 for file in $OMAKUB_PATH/migrations/*.sh; do