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>

builtin/fsck.c

@@ -630,7 +630,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 	struct alternate_object_database *alt;
 
 	errors_found = 0;
-	read_replace_refs = 0;
+	check_replace_refs = 0;
 
 	argc = parse_options(argc, argv, prefix, fsck_opts, fsck_usage, 0);
 

builtin/index-pack.c

@@ -1502,7 +1502,7 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
 	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);

builtin/pack-objects.c

@@ -2507,7 +2507,7 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		OPT_END(),
 	};
 
-	read_replace_refs = 0;
+	check_replace_refs = 0;
 
 	reset_pack_idx_option(&pack_idx_opts);
 	git_config(git_pack_config, NULL);