onemerge: make sure each merge in master..seen brings in only one topic

If a topic B is built on top of a merge of topic A into master, then
we should merge topic A first before topic B when preparing the
integration branches.  Merging topic B first would bring topic A
along with it, which is generally not a good idea if we consider A
and B as two separate topics.

onemerge.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+# between master..seen, are there merges that bring in 
+# more than one topic at a time?
+
+endpoint=${1-seen}
+
+tmp=/var/tmp/e.$$
+rm -f "$tmp.1" "$tmp.2" &&
+prev= &&
+trap 'rm -f "$tmp.*"' 0 || exit
+
+git rev-list --merges --first-parent master..$endpoint |
+while read commit
+do
+	# $tmp.1 has remaining topics after the merge we are looking at.
+	# $tmp.2 has remaining topics after the previous merge that is
+	# a descendant of the current merge.
+
+	git branch --list --no-merged "$commit" '??/*' >"$tmp.1"
+	if test -f "$tmp.2" && test -n "$prev"
+	then
+		cnt=$(comm -23 "$tmp.1" "$tmp.2" | wc -l)
+		if test $cnt != 1
+		then
+			echo Merges multiple topics
+			git show -s --format="* %s" "$prev"
+			comm -23 "$tmp.1" "$tmp.2"
+			exit
+
+		fi
+	fi
+	mv -f "$tmp.1" "$tmp.2"
+	prev=$commit
+done