fix git branch -m in presence of cross devices

fix git branch -m in presence of cross devices

When you have for example a bare repository stored on NFS, and that you
create new workdirs locally (using contrib's git-new-workdir), logs/refs
is a symlink to a different device. Hence when the reflogs are renamed,
all must happen below logs/refs or one gets cross device rename errors
like:

  git branch -m foo
  error: unable to move logfile logs/refs/heads/master to tmp-renamed-log: Invalid cross-device link
  fatal: Branch rename failed

The fix is hence to use logs/refs/.tmp-renamed-log as a temporary log
name, instead of just tmp-renamed-log.

Signed-off-by: Pierre Habouzit <madcoder@debian.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>