sync_client_software: fetch depth=1 and clean untracked files on upgrade

Replace the broken `git rebase HEAD^` with a proper shallow fetch:
- `git fetch --depth=1` limits local history to one commit, regardless of server history
- `git reset --hard FETCH_HEAD` syncs the working tree to the fetched tip
- `git clean -fd` removes untracked non-ignored files left by old versions
- `git gc --prune=now` immediately purges unreachable history objects

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

system_setup/sync_client_software.sh

@@ -51,8 +51,8 @@ if [[ ! -z "${REPO_URL}" ]]; then
         else
             # Doing upgrade, discarding all local changes frist (is more save than forced pull)
             echo "Checks have passed, we are now upgrading via git."
-            #Fetch latest Updates and remove all history
-            git fetch origin && git reset --hard origin/${REPO_BRANCH} && git rebase HEAD^
+            #Fetch latest commit only (depth=1), reset working tree, purge old history and untracked files
+            git fetch --depth=1 origin ${REPO_BRANCH} && git reset --hard FETCH_HEAD && git clean -fd && git gc --prune=now --quiet
             if [[ $? -ne 0 ]]; then
                 echo "Error: Failure while updating, will continue as is."
             fi