pretty.c: refactor trailer logic to `format_set_trailers_options()`

pretty.c: refactor trailer logic to `format_set_trailers_options()`

Refactored trailers formatting logic inside pretty.c to a new function
`format_set_trailers_options()`. This new function returns the non-zero
in case of unusual. The caller handles the non-zero by "goto trailers_out".

This change will allow us to reuse the same logic in other places.

Mentored-by: Christian Couder <redacted>
Mentored-by: Heba Waly <redacted>
Signed-off-by: Hariom Verma <redacted>
Signed-off-by: Junio C Hamano <redacted>