Correct builtin-fetch to handle + in refspecs

Correct builtin-fetch to handle + in refspecs

If we are fetching to a local reference (the so called peer_ref) and
the refspec that created this ref/peer_ref association had started
with '+' we are supposed to allow a non-fast-forward update during
fetch, even if --force was not supplied on the command line.  The
builtin-fetch implementation was not honoring this setting as it
was copied from the wrong struct ref instance.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>