Ensure new job IDs are never smaller than existing running jobs

This makes job IDs "monotone" in the sense that newly spawned jobs
always have larger IDs than existing jobs, as requested in #6053

tests/checks/job-ids.fish

@@ -29,6 +29,23 @@ jobs
 #CHECK: 2{{.*\t}}sleep 200 &
 #CHECK: 1{{.*\t}}sleep 100 &
 
+# Kill job 2; the next job should have job ID 4 because 3 is still in use (#6053).
+
+status job-control interactive
+command kill -9 $tokill[2]
+set -e tokill[2]
+status job-control full
+sleep 400 &
+set -g tokill $tokill $last_pid
+
+jobs
+
+#CHECK: Job	Group{{.*}}
+#CHECK: 4{{.*\t}}sleep 400 &
+#CHECK: 3{{.*\t}}sleep 300 &
+#CHECK: 1{{.*\t}}sleep 100 &
+
+
 status job-control interactive
 
 for pid in $tokill