t0090: make test pass with SHA-256

t0090: make test pass with SHA-256

One assertion of this test checks for a shrinking cache tree.  The
initial index contains a cache tree with two directory names but no
object ID, and the second index contains a cache tree with an object ID
but no directory name.

With SHA-1, the second index is smaller than the first, because the
directory information stored takes more than the 20 bytes of an SHA-1
hash, but with SHA-256, the hash is longer, and the test fails the
assertion that the second index is smaller than the first.

To address this issue, increase the length of the subdirectory name to
ensure that the cache tree does indeed shrink in size regardless of the
algorithm in use.

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