gpg-interface: move parse_signature() to where it should be

gpg-interface: move parse_signature() to where it should be

Our signed-tag objects set the standard format used by Git to store
GPG-signed payload (i.e. the payload followed by its detached
signature), and it made sense to have a helper to find the boundary
between the payload and its signature in tag.c back then.

Newer code added later to parse other kinds of objects that learned
to use the same format to store GPG-signed payload (e.g. signed
commits), however, kept using the helper from the same location.

Move it to gpg-interface; the helper is no longer about signed tag,
but it is how our code and data interact with GPG.

Signed-off-by: Junio C Hamano <gitster@pobox.com>