[CI/CD] Do a git fetch beofre checkout/rebase in sync-stable-branch.

.drone/sync-stable-branch.sh

@@ -7,12 +7,13 @@
 # Git configuration
 git remote rm origin
 git remote add origin git@git.platypush.tech:platypush/platypush.git
+git fetch origin
 
 # Merge and push to the `stable` branch
 git checkout stable || git checkout -b stable
-git rebase origin master
+git rebase master
 git push -u origin stable
-git checkout origin master
+git checkout master
 
 # Restore the original git configuration
 mv "$TMP_GIT_CONF" "$GIT_CONF"