interpret-trailers: add --parse convenience option

interpret-trailers: add --parse convenience option

The last few commits have added command line options that
can turn interpret-trailers into a parsing tool. Since
they'd most often be used together, let's provide a
convenient single option for callers to invoke this mode.

This is implemented as a callback rather than a boolean so
that its effect is applied immediately, as if those options
had been specified. Later options can then override them.
E.g.:

  git interpret-trailers --parse --no-unfold

would work.

Let's also update the documentation to make clear that this
parsing mode behaves quite differently than the normal
"add trailers to the input" mode.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>