test: add test for --use-mailmap option

The new option '--use-mailmap' can be used to make sure that mailmap
file is used to convert name when running log commands.

The test is simple and checks that the Author line
is correctly replaced when running log.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: apelisse

t/t4203-mailmap.sh

@@ -239,6 +239,20 @@ test_expect_success 'Log output (complex mapping)' '
 	test_cmp expect actual
 '
 
+cat >expect <<\EOF
+Author: CTO <cto@company.xx>
+Author: Santa Claus <santa.claus@northpole.xx>
+Author: Santa Claus <santa.claus@northpole.xx>
+Author: Other Author <other@author.xx>
+Author: Other Author <other@author.xx>
+Author: Some Dude <some@dude.xx>
+Author: A U Thor <author@example.com>
+EOF
+test_expect_success 'Log output with --use-mailmap' '
+	git log --use-mailmap | grep Author >actual &&
+	test_cmp expect actual
+'
+
 # git blame
 cat >expect <<\EOF
 ^OBJI (A U Thor     DATE 1) one