diff: correct newline in summary for renamed files

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 <redacted>
Helped-by: Jeff King <redacted>
Signed-off-by: Stefan Beller <redacted>
Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Junio C Hamano <redacted>