verify_signed_buffer: drop pbuf variable

verify_signed_buffer: drop pbuf variable

If our caller gave us a non-NULL gpg_status parameter, we
write the gpg status into their strbuf. If they didn't, then
we write it to a temporary local strbuf (since we still need
to look at it).  The variable "pbuf" adds an extra layer of
indirection so that the rest of the function can just access
whichever is appropriate.

However, the name "pbuf" isn't very descriptive, and it's
easy to get confused about what is supposed to be in it
(especially because we are reading both "status" and
"output" from gpg).

Rather than give it a more descriptive name, we can just use
gpg_status as our indirection pointer. Either it points to
the caller's input, or we can point it directly to our
temporary buffer.

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