commit: allow parsing arbitrary buffers with headers

commit: allow parsing arbitrary buffers with headers

Currently only commits are signed with headers.  However, in the future,
we'll also sign tags with headers as well.  Let's refactor out a
function called parse_buffer_signed_by_header which does exactly that.
In addition, since we'll want to sign things other than commits this
way, let's call the function sign_with_header instead of do_sign_commit.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>