gpg-interface: fix leak of strbufs in get_ssh_key_fingerprint()

gpg-interface: fix leak of strbufs in get_ssh_key_fingerprint()

We read stdout from gpg into a strbuf, then split it into a list of
strbufs, pull out one element, and return it. But we don't free either
the original stdout buffer, nor the list returned from strbuf_split().

This patch fixes both. Note that we have to detach the returned string
from its strbuf before calling strbuf_list_free(), as that would
otherwise throw it away.

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