Allow removal of "path" when "path/file" exists.

Allow removal of "path" when "path/file" exists.

When we used to have "path" as a file or a symlink, but now we
have "path/file" (or in general, have "path" as a directory), we
would want to remove "path" before adding "path/file".  The
logic in add_file_to_cache() only runs lstat() and does not
detect this case and fails to allow removing it in this case.
In the opposite case of having "path/file" in the index and
having "path" on the filesystem as a file or a symlink we do
allow removal of "path/file", so to be symmetric we should allow
it as well, without forcing the user to say --force-remove.

Signed-off-by: Junio C Hamano <junkio@cox.net>