update-submodules: attempt to fix checkout issue, properly exit on errors

A recent run failed to update luacov, but did not fail the workflow run:

https://github.com/DFHack/dfhack/runs/3298049090?check_suite_focus=true

```
+ git checkout dfhack
error: pathspec 'dfhack' did not match any file(s) known to git
```

.github/workflows/update-submodules.yml

@@ -28,4 +28,7 @@ jobs:
         git config user.email "63161697+DFHack-Urist@users.noreply.github.com"
         if bash -x ci/update-submodules.bash; then
           git push --recurse-submodules=check
+        elif ! git diff --exit-code HEAD; then
+          echo "update-submodules.bash failed to commit changes"
+          exit 1
         fi

ci/update-submodules.bash

@@ -21,6 +21,7 @@ write_msg ""
 cat ci/update-submodules.manifest | while read path branch; do
     cd "${git_root}/${path}"
     test "${git_root}" != "$(get_git_root)"
+    git fetch origin "${branch}:${branch}"
     git checkout "${branch}"
     git pull --ff-only
     cd "${git_root}"