prompt: clean up strbuf usage

prompt: clean up strbuf usage

The do_askpass function inherited a few bad habits from the
original git_getpass. One, there's no need to strbuf_reset a
buffer which was just initialized. And two, it's a good
habit to use strbuf_detach to claim ownership of a buffer's
string (even though in this case the owning buffer goes out
of scope, so it's effectively the same thing).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>