Upgrade test: fail when nothing was upgraded, and retry a lost push

A channel that holds the version the machine already has upgrades
nothing, and every other check passes on a machine that was only
rebooted: rc is in that state today. A twelfth check requires the
Omarchy package to have changed.

Two runs on different branches both push pictures to one branch. The
second to push was refused and its comment never written. The push is
now tried again on top of whatever got there first; each run's folder is
its own, so there is nothing to merge.

.github/workflows/upgrade-test.yml

@@ -155,20 +155,28 @@ jobs:
           set -euo pipefail
           [ -s evidence/before.png ] || [ -s evidence/after.png ] || { echo "no pictures to commit"; exit 0; }
           url=https://x-access-token:$GH_TOKEN@github.com/$GITHUB_REPOSITORY.git
+          dir=$GITHUB_RUN_ID-$GITHUB_RUN_ATTEMPT
           git init -q shots && cd shots
           git config user.name "github-actions[bot]"
           git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
-          if git fetch -q --depth 1 "$url" "$SHOTS_BRANCH" 2>/dev/null; then
-            git checkout -q -b "$SHOTS_BRANCH" FETCH_HEAD
-          else
-            git checkout -q --orphan "$SHOTS_BRANCH"
-          fi
-          dir=$GITHUB_RUN_ID-$GITHUB_RUN_ATTEMPT
-          mkdir -p "$dir"
-          cp ../evidence/*.png "$dir/"
-          git add "$dir"
-          git commit -q -m "Upgrade test $GITHUB_RUN_ID, attempt $GITHUB_RUN_ATTEMPT: $FROM to $TO"
-          git push -q "$url" "$SHOTS_BRANCH"
+          # Runs on other branches push here too. A push that loses the race
+          # is made again on top of whatever won: the folder is this run's own,
+          # so there is never anything to merge.
+          for attempt in 1 2 3 4 5; do
+            if git fetch -q --depth 1 "$url" "$SHOTS_BRANCH" 2>/dev/null; then
+              git checkout -q -B "$SHOTS_BRANCH" FETCH_HEAD
+            else
+              git checkout -q --orphan "$SHOTS_BRANCH" 2>/dev/null || true
+            fi
+            mkdir -p "$dir"
+            cp ../evidence/*.png "$dir/"
+            git add "$dir"
+            git commit -q -m "Upgrade test $GITHUB_RUN_ID, attempt $GITHUB_RUN_ATTEMPT: $FROM to $TO"
+            if git push -q "$url" "HEAD:refs/heads/$SHOTS_BRANCH"; then break; fi
+            [ "$attempt" -lt 5 ] || { echo "::error::could not push the pictures after 5 tries"; exit 1; }
+            echo "The push lost a race, trying again"
+            sleep $((attempt * 2))
+          done
           echo "base=https://raw.githubusercontent.com/$GITHUB_REPOSITORY/$(git rev-parse HEAD)/$dir" >>"$GITHUB_OUTPUT"
 
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