t3305: make hash size independent

t3305: make hash size independent

Instead of hard-coding 40-character shell patterns, use grep to
determine if all of the paths have either zero or one levels of fanout,
as appropriate.

Note that the final test is implicitly dependent on the hash algorithm.
Depending on the algorithm in use, the fanout may or may not completely
compress.  In its current state, this is not a problem, but it could be
if the hash algorithm changes again.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>