ref-filter: 'contents:trailers' show error if `:` is missing

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 <redacted>
Mentored-by: Heba Waly <redacted>
Helped-by: Eric Sunshine <redacted>
Signed-off-by: Hariom Verma <redacted>
Signed-off-by: Junio C Hamano <redacted>