patch-id: ignore newline at end of file in diff_flush_patch_id()

Whitespace is ignored when calculating patch IDs.  This is done by
removing all whitespace from diff lines before hashing them, including
a newline at the end of a file.  If that newline is missing, however,
diff reports that fact in a separate line containing "\ No newline at
end of file\n", and this marker is hashed like a context line.

This goes against our goal of making patch IDs independent of
whitespace.  Use the same heuristic that 2485eab55c (git-patch-id: do
not trip over "no newline" markers, 2011-02-17) added to git patch-id
instead and skip diff lines that start with a backslash and a space
and are longer than twelve characters.

Reported-by: Tilman Vogel <tilman.vogel@web.de>
Initial-test-by: Tilman Vogel <tilman.vogel@web.de>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -6044,6 +6044,8 @@ static void patch_id_consume(void *priv, char *line, unsigned long len)
 	struct patch_id_t *data = priv;
 	int new_len;
 
+	if (len > 12 && starts_with(line, "\\ "))
+		return;
 	new_len = remove_space(line, len);
 
 	the_hash_algo->update_fn(data->ctx, line, new_len);