t3700 (add): add two tests for testing add with submodules

The first test "git add should not go past gitlink boundaries" checks
that paths within a submodule added with 'git submodule add' cannot be
added to the index.  It passes because of treat_gitlink() in
builtin/add.c.

The second test "git add should not go past git repository boundaries"
checks that paths within a git repository in the worktree (not yet
added with 'git submodule add') cannot be added to the index.  It
fails because there is no existing code to check this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
add: refuse to add paths beyond repository boundaries

Currently, git add has the logic for refusing to add gitlinks using
treat_path(), which in turn calls check_path_for_gitlink().  However,
this only checks for an in-index submodule (or gitlink cache_entry).
A path inside a git repository in the worktree still adds fine, and
this is a bug.  The logic for denying it is very similar to denying
adding paths beyond symbolic links: die_if_path_beyond_symlink().
Follow its example and write a die_if_path_beyond_gitrepo() to fix
this bug.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>