for-each-ref: add split message parts to %(contents:*).

for-each-ref: add split message parts to %(contents:*).

The %(body) placeholder returns the whole body of a tag or
commit, including the signature. However, callers may want
to get just the body without signature, or just the
signature.

Rather than change the meaning of %(body), which might break
some scripts, this patch introduces a new set of
placeholders which break down the %(contents) placeholder
into its constituent parts.

[jk: initial patch by mg, rebased on top of my refactoring
and with tests by me]

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