apply: reject input that touches outside the working area

apply: reject input that touches outside the working area

By default, a patch that affects outside the working area (either a
Git controlled working tree, or the current working directory when
"git apply" is used as a replacement of GNU patch) is rejected as a
mistake (or a mischief).  Git itself does not create such a patch,
unless the user bends over backwards and specifies a non-standard
prefix to "git diff" and friends.

When `git apply` is used as a "better GNU patch", the user can pass
the `--unsafe-paths` option to override this safety check. This
option has no effect when `--index` or `--cached` is in use.

The new test was stolen from Jeff King with slight enhancements.
Note that a few new tests for touching outside the working area by
following a symbolic link are still expected to fail at this step,
but will be fixed in later steps.

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