diff: correct newline in summary for renamed files

In 146fdb0dfe (diff.c: emit_diff_symbol learns about DIFF_SYMBOL_SUMMARY,
2017-06-29), the conversion from direct printing to the symbol emission
dropped the new line character for renamed, copied and rewritten files.

Add the emission of a newline, add a test for this case.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -5251,6 +5251,7 @@ static void show_mode_change(struct diff_options *opt, struct diff_filepair *p,
 			strbuf_addch(&sb, ' ');
 			quote_c_style(p->two->path, &sb, NULL, 0);
 		}
+		strbuf_addch(&sb, '\n');
 		emit_diff_symbol(opt, DIFF_SYMBOL_SUMMARY,
 				 sb.buf, sb.len, 0);
 		strbuf_release(&sb);