pretty: allow %(trailers) options with explicit value

pretty: allow %(trailers) options with explicit value

In addition to old %(trailers:only) it is now allowed to write
%(trailers:only=yes)

By itself this only gives (the not quite so useful) possibility to have
users change their mind in the middle of a formatting
string (%(trailers:only=true,only=false)). However, it gives users the
opportunity to override defaults from future options.

Signed-off-by: Anders Waldenborg <anders@0x63.nu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>