ref-filter: hoist signature parsing

ref-filter: hoist signature parsing

When we parse a signature in the ref-filter code, we continually
increment the buffer pointer.  Hoist the signature parsing above the
blank line delimiting headers and body so we can find the signature when
using a header to sign the buffer.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>