pretty-format %(trailers): fix broken standalone "valueonly"

pretty-format %(trailers): fix broken standalone "valueonly"

Fix %(trailers:valueonly) being a noop due to on overly eager
optimization in format_trailer_info() which skips custom formatting if
no custom options are given.

When "valueonly" was added in d9b936db522 (pretty: add support for
"valueonly" option in %(trailers), 2019-01-28) we forgot to add it to
the list of options that optimization checks for. See e.g. the
addition of "key" in 250bea0c165 (pretty: allow showing specific
trailers, 2019-01-28) for a similar change where this wasn't missed.

Thus the "valueonly" option in "%(trailers:valueonly)" was a noop and
the output was equivalent to that of a plain "%(trailers)". This
wasn't caught because the tests for it always combined it with other
options.

Fix the bug by adding !opts->value_only to the list. I initially
attempted to make this more future-proof by setting a flag if we got
to ":" in "%(trailers:" in format_commit_one() in pretty.c. However,
"%(trailers:" is also parsed in trailers_atom_parser() in
ref-filter.c.

There is an outstanding patch[1] unify those two, and such a fix, or
other future-proofing, such as changing "process_trailer_options"
flags into a bitfield, would conflict with that effort. Let's instead
do the bare minimum here as this aspect of trailers is being actively
worked on by another series.

Let's also test for a plain "valueonly" without any other options, as
well as "separator". All the other existing options on the pretty.c
path had tests where they were the only option provided. I'm also
keeping a sanity test for "%(trailers:)" being the same as
"%(trailers)". There's no reason to suspect it wouldn't be in the
current implementation, but let's keep it in the interest of black box
testing.

1. https://lore.kernel.org/git/pull.726.git.1599335291.gitgitgadget@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>