tag: fix sign_buffer() call to create a signed tag

The command "git tag -s" internally calls sign_buffer() to make a
cryptographic signature using the chosen backend like GPG and SSH.
The internal helper functions used by "git tag" implementation seem
to use a "negative return values are errors, zero or positive return
values are not" convention, and there are places (e.g., verify_tag()
that calls gpg_verify_tag()) that these internal helper functions
translate return values that signal errors to conform to this
convention, but do_sign() that calls sign_buffer() forgets to do so.

Fix it, so that a failed call to sign_buffer() that can return the
exit status from pipe_command() will not be overlooked.

Reported-by: Sergey Kosukhin <skosukhin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/tag.c

@@ -176,7 +176,7 @@ static int verify_tag(const char *name, const char *ref UNUSED,
 
 static int do_sign(struct strbuf *buffer)
 {
-	return sign_buffer(buffer, buffer, get_signing_key());
+	return sign_buffer(buffer, buffer, get_signing_key()) ? -1 : 0;
 }
 
 static const char tag_template[] =