ci: Use ... diff in list-changed-projects.sh (#25885)

The base ref passed from GitHub is the tip of the target branch, not the
merge-base with the current PR. `git diff base..head` will wind up
showing a diff that reverses any changes between the merge-base and the
passed base; the behavior we want is `git diff base...head`.

.github/files/list-changed-projects.sh

@@ -19,7 +19,7 @@ ARGS=( --add-dependents )
 
 if [[ "${GITHUB_EVENT_NAME:?}" == "pull_request" ]]; then
 	[[ -f "${GITHUB_EVENT_PATH:?}" ]] || die "GITHUB_EVENT_PATH file $GITHUB_EVENT_PATH does not exist"
-	DIFF="$(jq -r '"\( .pull_request.base.sha )..\( .pull_request.head.sha )"' "$GITHUB_EVENT_PATH")"
+	DIFF="$(jq -r '"\( .pull_request.base.sha )...\( .pull_request.head.sha )"' "$GITHUB_EVENT_PATH")"
 	debug "GITHUB_EVENT_NAME is pull_request, checking diff from $DIFF"
 	ARGS+=( --verbose "--git-changed=$DIFF" )
 elif [[ "${GITHUB_EVENT_NAME:?}" == "push" ]]; then