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));
 	}