The placeholders can be grouped into three kinds:
 * literals
 * affecting formatting of later placeholders
 * expanding to information in commit

Also change the list to a definition list (using '::')

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

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>

No functional change intended.

This change may not seem useful on its own, but upcoming commits will do
memory allocation in there, and a single return path makes deallocation
easier.

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

Adds a new "key=X" option to "%(trailers)" which will cause it to only
print trailer lines which match any of the specified keys.

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

With the new "key=" option to %(trailers) it often makes little sense to
show the key, as it by definition already is knows which trailer is
printed there. This new "valueonly" option makes it omit the key when
printing trailers.

E.g.:
 $ git show -s --pretty='%s%n%(trailers:key=Signed-off-by,valueonly)' aaaa881
will show:
 > upload-pack: fix broken if/else chain in config callback
 > Jeff King <peff@peff.net>
 > Junio C Hamano <gitster@pobox.com>

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

Expanding '%n' and '%xNN' is generic functionality, so extract that from
the pretty.c formatter into a callback that can be reused.

No functional change intended

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

By default trailer lines are terminated by linebreaks ('\n'). By
specifying the new 'separator' option they will instead be separated by
user provided string and have separator semantics rather than terminator
semantics. The separator string can contain the literal formatting codes
%n and %xNN allowing it to be things that are otherwise hard to type
such as %x00, or comma and end-parenthesis which would break parsing.

E.g:
 $ git log --pretty='%(trailers:key=Reviewed-by,valueonly,separator=%x00)'

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