send-email: implement sendmail aliases line continuation support

send-email: implement sendmail aliases line continuation support

Logical lines in sendmail aliases files can be spread over multiple
physical lines[1]. A line beginning with whitespace is folded into the
preceding line. A line ending with '\' consumes the following line.

[1]: https://www.freebsd.org/cgi/man.cgi?query=aliases&sektion=5

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>