t5537: make hash size independent

t5537: make hash size independent

This test modifies a pkt-line stream with sed to change a line with
"shallow" to "unshallow".  However, this modification is dependent on
the size of the hash in use; with SHA-256, the pkt-line length is
different, leading to the sed command having no effect.

Use test_oid_cache to specify the correct values for each hash so that
the test continues to work.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>