There are several places in our testsuite where we want to insert a
slash after an object ID to make it into a path we can reference under
.git/objects, and we have various ways of doing so. Add a helper to
provide a standard way of doing this that works for all size hashes.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>