dgit: WWW::Curl Set $response_body to '' explicitly

dgit: WWW::Curl Set $response_body to '' explicitly

check_for_git calls url_fetch with a WWW::Curl parameter
CURLOPT_NOBODY which suppresses saving the body.  The result is that
CURLOPT_WRITEDATA does not update $response_body.  Then, even on
success, url_fetch returns undef (which ought to mean 404).

Strictly this is a bit sneaky of check_for_git.  But, rather than
trying to make this interface more formal, just make url_fetch set
$response_body to ''.  It does return this except in the success case,
so the error cases are still handled right.

Closes: #934126
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>