tests: add checking that combine-diff emits only correct paths

where "correct paths" stands for paths that are different to all
parents.

Up until now, we were testing combined diff only on one file, or on
several files which were all different (t4038-diff-combined.sh).

As recent thinko in "simplify intersect_paths() further" showed, and
also, since we are going to rework code for finding paths different to
all parents, lets write at least basic tests.
Signed-off-by: NKirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4057-diff-combined-paths.sh

+#!/bin/sh
+
+test_description='combined diff show only paths that are different to all parents'
+
+. ./test-lib.sh
+
+# verify that diffc.expect matches output of
+# `git diff -c --name-only HEAD HEAD^ HEAD^2`
+diffc_verify () {
+	git diff -c --name-only HEAD HEAD^ HEAD^2 >diffc.actual &&
+	test_cmp diffc.expect diffc.actual
+}
+
+test_expect_success 'trivial merge - combine-diff empty' '
+	for i in $(test_seq 1 9)
+	do
+		echo $i >$i.txt	&&
+		git add $i.txt
+	done &&
+	git commit -m "init" &&
+	git checkout -b side &&
+	for i in $(test_seq 2 9)
+	do
+		echo $i/2 >>$i.txt
+	done &&
+	git commit -a -m "side 2-9" &&
+	git checkout master &&
+	echo 1/2 >1.txt &&
+	git commit -a -m "master 1" &&
+	git merge side &&
+	>diffc.expect &&
+	diffc_verify
+'
+
+
+test_expect_success 'only one trully conflicting path' '
+	git checkout side &&
+	for i in $(test_seq 2 9)
+	do
+		echo $i/3 >>$i.txt
+	done &&
+	echo "4side" >>4.txt &&
+	git commit -a -m "side 2-9 +4" &&
+	git checkout master &&
+	for i in $(test_seq 1 9)
+	do
+		echo $i/3 >>$i.txt
+	done &&
+	echo "4master" >>4.txt &&
+	git commit -a -m "master 1-9 +4" &&
+	test_must_fail git merge side &&
+	cat <<-\EOF >4.txt &&
+	4
+	4/2
+	4/3
+	4master
+	4side
+	EOF
+	git add 4.txt &&
+	git commit -m "merge side (2)" &&
+	echo 4.txt >diffc.expect &&
+	diffc_verify
+'
+
+test_expect_success 'merge introduces new file' '
+	git checkout side &&
+	for i in $(test_seq 5 9)
+	do
+		echo $i/4 >>$i.txt
+	done &&
+	git commit -a -m "side 5-9" &&
+	git checkout master &&
+	for i in $(test_seq 1 3)
+	do
+		echo $i/4 >>$i.txt
+	done &&
+	git commit -a -m "master 1-3 +4hello" &&
+	git merge side &&
+	echo "Hello World" >4hello.txt &&
+	git add 4hello.txt &&
+	git commit --amend &&
+	echo 4hello.txt >diffc.expect &&
+	diffc_verify
+'
+
+test_expect_success 'merge removed a file' '
+	git checkout side &&
+	for i in $(test_seq 5 9)
+	do
+		echo $i/5 >>$i.txt
+	done &&
+	git commit -a -m "side 5-9" &&
+	git checkout master &&
+	for i in $(test_seq 1 3)
+	do
+		echo $i/4 >>$i.txt
+	done &&
+	git commit -a -m "master 1-3" &&
+	git merge side &&
+	git rm 4.txt &&
+	git commit --amend &&
+	echo 4.txt >diffc.expect &&
+	diffc_verify
+'
+
+test_done