Diff: teach get_blob_content to show a submodule as text

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.

src/diff_output.c

@@ -212,6 +212,22 @@ static int get_blob_content(
 	if (git_oid_iszero(&file->oid))
 		return 0;
 
+	if (file->mode == GIT_FILEMODE_COMMIT)
+	{
+		char oidstr[GIT_OID_HEXSZ+1];
+		git_buf content = GIT_BUF_INIT;
+
+		git_oid_fmt(oidstr, &file->oid);
+		oidstr[GIT_OID_HEXSZ] = 0;
+		git_buf_printf(&content, "Subproject commit %s\n", oidstr );
+
+		map->data = git_buf_detach(&content);
+		map->len = strlen(map->data);
+
+		file->flags |= GIT_DIFF_FILE_FREE_DATA;
+		return 0;
+	}
+
 	if (!file->size) {
 		git_odb *odb;
 		size_t len;