csum-file interface updates: return resulting SHA1

Also, make the writing of the SHA1 as a end-header be conditional: not
every user will necessarily want to write the SHA1 to the file itself,
even though current users do (but we migh end up using the same helper
functions for the object files themselves, that don't do this).

This also makes the packed index file contain the SHA1 of the packed
data file at the end (just before its own SHA1).  That way you can
validate the pairing of the two if you want to.

unpack-objects.c

@@ -74,9 +74,10 @@ static int check_index(void)
 	 * Total size:
 	 *  - 256 index entries 4 bytes each
 	 *  - 24-byte entries * nr (20-byte sha1 + 4-byte offset)
+	 *  - 20-byte SHA1 of the packfile
 	 *  - 20-byte SHA1 file checksum
 	 */
-	if (index_size != 4*256 + nr * 24 + 20)
+	if (index_size != 4*256 + nr * 24 + 20 + 20)
 		return error("wrong index file size");
 
 	nr_entries = nr;