rebase: be stricter when reading state files containing oids

rebase: be stricter when reading state files containing oids

The state files for 'onto' and 'orig_head' should contain a full hex
oid, change the reading functions from get_oid() to get_oid_hex() to
reflect this. They should also name commits and not tags so add and use
a function that looks up a commit from an oid like
lookup_commit_reference() but without dereferencing tags.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>