t8008: rely on rev-parse'd HEAD instead of sha1 value

t8008: rely on rev-parse'd HEAD instead of sha1 value

Remove hard coded sha1 values, obtain the values using
'git rev-parse HEAD' which should be future proof regardless
of the hash function used.

Additionally future-proof the test by hard coding the
abbreviation length of the hash.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>