Since commit eeaa1c3a

 (Separate text from footer in notification emails,
2020-01-04), information about unsubscribing from notifications is added
in a signature block. However, the code to format the email body trimmed
the RFC 3676 signature delimiter, replacing "-- " by "--". Fix this by
adding a special case for signature delimiters.

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>