fast-import: tighten M 040000 syntax

fast-import: tighten M 040000 syntax

When tree_content_set() is asked to modify the path "foo/bar/",
it first recurses like so:

	tree_content_set(root, "foo/bar/", sha1, S_IFDIR) ->
	 tree_content_set(root:foo, "bar/", ...) ->
	  tree_content_set(root:foo/bar, "", ...)

And as a side-effect of 2794ad5 (fast-import: Allow filemodify to set
the root, 2010-10-10), this last call is accepted and changes
the tree entry for root:foo/bar to refer to the specified tree.

That seems safe enough but let's reject the new syntax (we never meant
to support it) and make it harder for frontends to introduce pointless
incompatibilities with git fast-import 1.7.3.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>