builtin/clone.c: disallow `--local` clones with symlinks

When cloning a repository with `--local`, Git relies on either making a hardlink or copy to every file in the "objects" directory of the source repository. This is done through the callpath `cmd_clone()` -> `clone_local()` -> `copy_or_link_directory()`. The way this optimization works is by enumerating every file and directory recursively in the source repository's `$GIT_DIR/objects` directory, and then either making a copy or hardlink of each file. The only exception to this rule is when copying the "alternates" file, in which case paths are rewritten to be absolute before writing a new "alternates" file in the destination repo. One quirk of this implementation is that it dereferences symlinks when cloning. This behavior was most recently modified in 36596fd2df (clone: better handle symlinked files at .git/objects/, 2019-07-10), which attempted to support `--local` clones of repositories with symlinks in their objects directory in a platform-independent way. Unfortunately, this behavior of dereferencing symlinks (that is, creating a hardlink or copy of the source's link target in the destination repository) can be used as a component in attacking a victim by inadvertently exposing the contents of file stored outside of the repository. Take, for example, a repository that stores a Dockerfile and is used to build Docker images. When building an image, Docker copies the directory contents into the VM, and then instructs the VM to execute the Dockerfile at the root of the copied directory. This protects against directory traversal attacks by copying symbolic links as-is without dereferencing them. That is, if a user has a symlink pointing at their private key material (where the symlink is present in the same directory as the Dockerfile, but the key itself is present outside of that directory), the key is unreadable to a Docker image, since the link will appear broken from the container's point of view. This behavior enables an attack whereby a victim is convinced to clone a repository containing an embedded submodule (with a URL like "file:///proc/self/cwd/path/to/submodule") which has a symlink pointing at a path containing sensitive information on the victim's machine. If a user is tricked into doing this, the contents at the destination of those symbolic links are exposed to the Docker image at runtime. One approach to preventing this behavior is to recreate symlinks in the destination repository. But this is problematic, since symlinking the objects directory are not well-supported. (One potential problem is that when sharing, e.g. a "pack" directory via symlinks, different writers performing garbage collection may consider different sets of objects to be reachable, enabling a situation whereby garbage collecting one repository may remove reachable objects in another repository). Instead, prohibit the local clone optimization when any symlinks are present in the `$GIT_DIR/objects` directory of the source repository. Users may clone the repository again by prepending the "file://" scheme to their clone URL, or by adding the `--no-local` option to their `git clone` invocation. The directory iterator used by `copy_or_link_directory()` must no longer dereference symlinks (i.e., it *must* call `lstat()` instead of `stat()` in order to discover whether or not there are symlinks present). This has no bearing on the overall behavior, since we will immediately `die()` on encounter a symlink. Note that t5604.33 suggests that we do support local clones with symbolic links in the source repository's objects directory, but this was likely unintentional, or at least did not take into consideration the problem with sharing parts of the objects directory with symbolic links at the time. Update this test to reflect which options are and aren't supported. Helped-by: Johannes Schindelin <Johannes.Schindelin@gmx.de> Signed-off-by: Taylor Blau <me@ttaylorr.com>