refs/files-backend: convert many internals to struct object_id

Convert many of the internals of the files backend to use struct
object_id.  Avoid converting public APIs (except one change to
refs/ref-cache.c) to limit the scope of the changes.

Convert one use of get_sha1_hex to parse_oid_hex, and rely on the fact
that a strbuf will be NUL-terminated and that parse_oid_hex will fail on
truncated input to avoid the need to check the length.

This is a requirement to convert parse_object later on.

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

refs/ref-cache.h

@@ -185,7 +185,7 @@ struct ref_entry *create_dir_entry(struct ref_cache *cache,
 				   int incomplete);
 
 struct ref_entry *create_ref_entry(const char *refname,
-				   const unsigned char *sha1, int flag,
+				   const struct object_id *oid, int flag,
 				   int check_name);
 
 /*