apply: do not touch a file beyond a symbolic link

Because Git tracks symbolic links as symbolic links, a path that
has a symbolic link in its leading part (e.g. path/to/dir/file,
where path/to/dir is a symbolic link to somewhere else, be it
inside or outside the working tree) can never appear in a patch
that validly applies, unless the same patch first removes the
symbolic link to allow a directory to be created there.

Detect and reject such a patch.

Things to note:

 - Unfortunately, we cannot reuse the has_symlink_leading_path()
   from dir.c, as that is only about the working tree, but "git
   apply" can be told to apply the patch only to the index or to
   both the index and to the working tree.

 - We cannot directly use has_symlink_leading_path() even when we
   are applying only to the working tree, as an early patch of a
   valid input may remove a symbolic link path/to/dir and then a
   later patch of the input may create a path path/to/dir/file, but
   "git apply" first checks the input without touching either the
   index or the working tree.  The leading symbolic link check must
   be done on the interim result we compute in-core (i.e. after the
   first patch, there is no path/to/dir symbolic link and it is
   perfectly valid to create path/to/dir/file).

   Similarly, when an input creates a symbolic link path/to/dir and
   then creates a file path/to/dir/file, we need to flag it as an
   error without actually creating path/to/dir symbolic link in the
   filesystem.

Instead, for any patch in the input that leaves a path (i.e. a non
deletion) in the result, we check all leading paths against the
resulting tree that the patch would create by inspecting all the
patches in the input and then the target of patch application
(either the index or the working tree).

This way, we catch a mischief or a mistake to add a symbolic link
path/to/dir and a file path/to/dir/file at the same time, while
allowing a valid patch that removes a symbolic link path/to/dir and
then adds a file path/to/dir/file.

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

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