t6023: merge-file fails to output anything for a degenerate merge

In the case that merge-file is passed three files with identical
contents it wipes the contents of the output file instead of
leaving it unchanged.

Althought merge-file is porcelain and this will never happen in
normal usage, it is still wrong.

Signed-off-by: Charles Bailey <charles@hashpling.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6023-merge-file.sh

@@ -54,6 +54,12 @@ deduxit me super semitas jusitiae,
 EOF
 printf "propter nomen suum." >> new4.txt
 
+test_expect_failure 'merge with no changes' '
+	cp orig.txt test.txt &&
+	git merge-file test.txt orig.txt orig.txt &&
+	test_cmp test.txt orig.txt
+'
+
 cp new1.txt test.txt
 test_expect_success "merge without conflict" \
 	"git merge-file test.txt orig.txt new2.txt"