gpg-interface: improve interface for parsing tags

We have a function which parses a buffer with a signature at the end,
parse_signature, and this function is used for signed tags.  However,
we'll need to store values for multiple algorithms, and we'll do this by
using a header for the non-default algorithm.

Adjust the parse_signature interface to store the parsed data in two
strbufs and turn the existing function into parse_signed_buffer.  The
latter is still used in places where we know we always have a signed
buffer, such as push certs.

Adjust all the callers to deal with this new interface.

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

gpg-interface.h

@@ -37,13 +37,20 @@ struct signature_check {
 
 void signature_check_clear(struct signature_check *sigc);
 
+/*
+ * Look at a GPG signed tag object.  If such a signature exists, store it in
+ * signature and the signed content in payload.  Return 1 if a signature was
+ * found, and 0 otherwise.
+ */
+int parse_signature(const char *buf, size_t size, struct strbuf *payload, struct strbuf *signature);
+
 /*
  * Look at GPG signed content (e.g. a signed tag object), whose
  * payload is followed by a detached signature on it.  Return the
  * offset where the embedded detached signature begins, or the end of
  * the data when there is no such signature.
  */
-size_t parse_signature(const char *buf, size_t size);
+size_t parse_signed_buffer(const char *buf, size_t size);
 
 /*
  * Create a detached signature for the contents of "buffer" and append