rename read_replace_refs to check_replace_refs

The semantics of this flag was changed in commit

    e1111cef23 inline lookup_replace_object() calls

but wasn't renamed at the time to minimize code churn.  Rename it now,
and add a comment explaining its use.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 2f37a38..a6b1c17 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -1502,7 +1502,7 @@
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage(index_pack_usage);
 
-	read_replace_refs = 0;
+	check_replace_refs = 0;
 
 	reset_pack_idx_option(&opts);
 	git_config(git_index_pack_config, &opts);