In a working tree prepared in new-workdir (in contrib/), some files in .git/
directory are symbolic links to the original repository.  The usual sequence of
lock-write-rename would break the symbolic link.

Ideally we should resolve relative symbolic link with maxdepth, but I do not
want to risk too elaborate patch before 1.5.3 release, so this is a minimum
and trivially obvious fix.  new-workdir creates its symbolic links absolute,
and does not link from a symlinked workdir, so this fix should suffice for now.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>