Add a demonstration/test of customized merge.

This demonstrates how the new low-level per-path merge backends,
union and ours, work, and shows how they are controlled by the
gitattribute mechanism.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

t/t6026-merge-attr.sh

+#!/bin/sh
+#
+# Copyright (c) 2007 Junio C Hamano
+#
+
+test_description='per path merge controlled by merge attribute'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+
+	for f in text binary union
+	do
+		echo Initial >$f && git add $f || break
+	done &&
+	test_tick &&
+	git commit -m Initial &&
+
+	git branch side &&
+	for f in text binary union
+	do
+		echo Master >>$f && git add $f || break
+	done &&
+	test_tick &&
+	git commit -m Master &&
+
+	git checkout side &&
+	for f in text binary union
+	do
+		echo Side >>$f && git add $f || break
+	done &&
+	test_tick &&
+	git commit -m Side
+
+'
+
+test_expect_success merge '
+
+	{
+		echo "binary -merge"
+		echo "union merge=union"
+	} >.gitattributes &&
+
+	if git merge master
+	then
+		echo Gaah, should have conflicted
+		false
+	else
+		echo Ok, conflicted.
+	fi
+'
+
+test_expect_success 'check merge result in index' '
+
+	git ls-files -u | grep binary &&
+	git ls-files -u | grep text &&
+	! (git ls-files -u | grep union)
+
+'
+
+test_expect_success 'check merge result in working tree' '
+
+	git cat-file -p HEAD:binary >binary-orig &&
+	grep "<<<<<<<" text &&
+	cmp binary-orig binary &&
+	! grep "<<<<<<<" union &&
+	grep Master union &&
+	grep Side union
+
+'
+
+test_done