gh-36632: .ci/merge-fixes.sh: Reduce output
    
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We suppress long output (https://github.com/sagemath/sage/actions/runs/6
738804468/job/18319117745?pr=36508#step:3:11) and scary-looking but
harmless error messages (https://github.com/sagemath/sage/actions/runs/6
738804468/job/18319117745?pr=36508#step:3:1114).

Example of a quieter run with this change: https://github.com/sagemath/s
age/actions/runs/6739070170/job/18319882833?pr=36632#step:3:19

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URL: #36632
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee

Author: Release Manager

.ci/merge-fixes.sh

@@ -14,10 +14,10 @@ else
     git tag -f test_base
     git commit -q -m "Uncommitted changes" --no-allow-empty -a
     for a in $PRs; do
+        git fetch --unshallow --all > /dev/null 2>&1 && echo "Unshallowed."
         echo "::group::Merging PR https://github.com/$REPO/pull/$a"
         git tag -f test_head
         $GH pr checkout -b pr-$a $a
-        git fetch --unshallow --all
         git checkout -q test_head
         if git merge --no-edit --squash -q pr-$a; then
             echo "::endgroup::"