odb: rename repo_read_object_file()

Rename `repo_read_object_file()` to `odb_read_object()` to match other
functions related to the object database and our modern coding
guidelines.

Introduce a compatibility wrapper so that any in-flight topics will
continue to compile.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -1931,8 +1931,8 @@ static int grep_source_load_oid(struct grep_source *gs)
 {
 	enum object_type type;
 
-	gs->buf = repo_read_object_file(gs->repo, gs->identifier, &type,
-					&gs->size);
+	gs->buf = odb_read_object(gs->repo->objects, gs->identifier,
+				  &type, &gs->size);
 	if (!gs->buf)
 		return error(_("'%s': unable to read %s"),
 			     gs->name,