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) [*1*], 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.

[Reference]
*1* http://thread.gmane.org/gmane.linux.kernel/297998/focus=1383

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

gpg-interface.c

@@ -7,6 +7,9 @@
 static char *configured_signing_key;
 static const char *gpg_program = "gpg";
 
+#define PGP_SIGNATURE "-----BEGIN PGP SIGNATURE-----"
+#define PGP_MESSAGE "-----BEGIN PGP MESSAGE-----"
+
 void signature_check_clear(struct signature_check *sigc)
 {
 	free(sigc->payload);
@@ -57,6 +60,24 @@ void parse_gpg_output(struct signature_check *sigc)
 	}
 }
 
+/*
+ * 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, unsigned long size)
+{
+	char *eol;
+	size_t len = 0;
+	while (len < size && !starts_with(buf + len, PGP_SIGNATURE) &&
+			!starts_with(buf + len, PGP_MESSAGE)) {
+		eol = memchr(buf + len, '\n', size - len);
+		len += eol ? eol - (buf + len) + 1 : size - len;
+	}
+	return len;
+}
+
 void set_signing_key(const char *key)
 {
 	free(configured_signing_key);