t1404: demonstrate a bug resolving references

t1404: demonstrate a bug resolving references

Add some tests checking that it is possible to work with a reference
even if there is an empty directory where the loose ref would be stored.

One of the new tests demonstrates a bug that has been with us since at
least 2.5.0--single reference lookup gives up when it sees the
directory, even if the reference exists as a packed ref. This probably
hasn't been reported before because Git usually cleans up empty
directories when packing references.

This bug will be fixed shortly.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>