tar-tree: add the "tar.umask" config option

tar-tree: add the "tar.umask" config option

By default, git-tar-tree(1) sets file and directories modes to 0666
or 0777. While this is both useful and acceptable for projects such
as the Linux Kernel, it might be excessive for other projects. With
this variable, it becomes possible to tell git-tar-tree(1) to apply
a specific umask to the modes above. The special value "user"
indicates that the user's current umask will be used. This should be
enough for most projects, as it will lead to the same permissions as
git-checkout(1) would use. The default value remains 0, which means
world read-write.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Acked-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>