Fix '\ No newline...' annotation in rewrite diffs

When a file that ends with an incomplete line is expressed as a
complete rewrite with the -B option, git diff incorrectly
appends the incomplete line indicator "\ No newline at end of
file" after such a line, rather than writing it on a line of its
own (the output codepath for normal output without -B does not
have this problem).  Add a LF after the incomplete line before
writing the "\ No newline ..." out to fix this.

Add a couple of tests to confirm that the indicator comment is
generated on its own line in both plain diff and rewrite mode.

Signed-off-by: Adam Butcher <dev.lists@jessamine.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -574,6 +574,7 @@ static void emit_rewrite_lines(struct emit_callback *ecb,
 	if (!endp) {
 		const char *plain = diff_get_color(ecb->color_diff,
 						   DIFF_PLAIN);
+		putc('\n', ecb->opt->file);
 		emit_line_0(ecb->opt, plain, reset, '\\',
 			    nneof, strlen(nneof));
 	}

t/t4022-diff-rewrite.sh

@@ -66,5 +66,35 @@ test_expect_success 'suppress deletion diff with -B -D' '
 	grep -v "Linus Torvalds" actual
 '
 
+test_expect_success 'prepare a file that ends with an incomplete line' '
+	test_seq 1 99 >seq &&
+	printf 100 >>seq &&
+	git add seq &&
+	git commit seq -m seq
+'
+
+test_expect_success 'rewrite the middle 90% of sequence file and terminate with newline' '
+	test_seq 1 5 >seq &&
+	test_seq 9331 9420 >>seq &&
+	test_seq 96 100 >>seq
+'
+
+test_expect_success 'confirm that sequence file is considered a rewrite' '
+	git diff -B seq >res &&
+	grep "dissimilarity index" res
+'
+
+test_expect_success 'no newline at eof is on its own line without -B' '
+	git diff seq >res &&
+	grep "^\\\\ " res &&
+	! grep "^..*\\\\ " res
+'
+
+test_expect_success 'no newline at eof is on its own line with -B' '
+	git diff -B seq >res &&
+	grep "^\\\\ " res &&
+	! grep "^..*\\\\ " res
+'
+
 test_done