git-tag: Do not assume the working tree root is writable. This is a long overdue companion commit that fixed git-commit (Santi's f8e2c54c9a17af3319e96db1d9e97ace36ae6831). Having the temporary files in the working tree root when making tags is not as bad because it does not involve 'git status' as the git-commit case, but this makes things more consistent. Signed-off-by: Junio C Hamano <junkio@cox.net>