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>

commit.h

@@ -64,6 +64,19 @@ enum decoration_type {
 void add_name_decoration(enum decoration_type type, const char *name, struct object *obj);
 const struct name_decoration *get_name_decoration(const struct object *obj);
 
+/*
+ * Look up commit named by "oid" respecting replacement objects.
+ * Returns NULL if "oid" is not a commit or does not exist.
+ */
+struct commit *lookup_commit_object(struct repository *r, const struct object_id *oid);
+
+/*
+ * Look up commit named by "oid" without replacement objects or
+ * checking for object existence. Returns the requested commit if it
+ * is found in the object cache, NULL if "oid" is in the object cache
+ * but is not a commit and a newly allocated unparsed commit object if
+ * "oid" is not in the object cache.
+ */
 struct commit *lookup_commit(struct repository *r, const struct object_id *oid);
 struct commit *lookup_commit_reference(struct repository *r,
 				       const struct object_id *oid);