ref-filter: 'contents:trailers' show error if `:` is missing
The 'contents' atom does not show any error if used with 'trailers'
atom and colon is missing before trailers arguments.
e.g %(contents:trailersonly) works, while it shouldn't.
It is definitely not an expected behavior.
Let's fix this bug.
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Heba Waly <heba.waly@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Hariom Verma <hariom18599@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>