http-push: fail when info/refs exists and is already locked

http-push: fail when info/refs exists and is already locked

Failing instead of silently not updating remote refs makes the things
clearer for the user when trying to push on a repository while another
person do (or while a dandling locks are waiting for a 10 minutes
timeout).

When silently not updating remote refs, the user does not even know
that git has pushed the objects but leaved the refs as they were
before (e.g. a new bunch of commits on branch "master" is uploaded,
however the branch by itsel still points on the previous head commit).

Signed-off-by: Junio C Hamano <gitster@pobox.com>