gpg: close stderr once finished with it in verify_signed_buffer()

Failing to close the stderr pipe in verify_signed_buffer() causes
git to run out of file descriptors if there are many calls to
verify_signed_buffer(). An easy way to trigger this is to run

 git log --show-signature --merges | grep "key"

on the linux kernel git repo. Eventually it will fail with

 error: cannot create pipe for gpg: Too many open files
 error: could not run gpg.

Close the stderr pipe so that this can't happen.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gpg-interface.c

@@ -128,8 +128,10 @@ int verify_signed_buffer(const char *payload, size_t payload_size,
 	write_in_full(gpg.in, payload, payload_size);
 	close(gpg.in);
 
-	if (gpg_output)
+	if (gpg_output) {
 		strbuf_read(gpg_output, gpg.err, 0);
+		close(gpg.err);
+	}
 	ret = finish_command(&gpg);
 
 	unlink_or_warn(path);