diff_output.c:get_blob_content used to try to read the submodule commit
as a blob in the superproject's odb. Of course it cannot find it and
errors out with GIT_ENOTFOUND, implcitly terminating the whole diff
output.

This patch teaches it to create a text that describes the submodule
instead. The text looks like:

	Subproject commit <SHA1>\n

which is what git.git does, too.