replace-object: eliminate replace objects prepared flag

Make the oidmap a pointer.

That way we eliminate the need for the global boolean
variable 'replace_object_prepared' as we can put this information
into the pointer being NULL or not.

Another advantage of this is that we would more quickly catch
code that tries to access replace-map without initializing it.

This also allows the '#include "oidmap.h"' introduced in a previous
patch to be replaced by the forward declaration of 'struct oidmap;'.
Keeping the type opaque discourages circumventing accessor functions;
not dragging in other headers avoids some compile time overhead.

One disadvantage of this is change is performance as we need to
pay the overhead for a malloc. The alternative of moving the
global variable into the object store is less modular code.

Helped-by: René Scharfe <l.s.r@web.de>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object-store.h

@@ -99,7 +99,7 @@ struct raw_object_store {
 	 * Objects that should be substituted by other objects
 	 * (see git-replace(1)).
 	 */
-	struct oidmap replace_map;
+	struct oidmap *replace_map;
 
 	/*
 	 * private data

replace_object.c

@@ -25,7 +25,7 @@ static int register_replace_ref(const char *refname,
 	oidcpy(&repl_obj->replacement, oid);
 
 	/* Register new object */
-	if (oidmap_put(&the_repository->objects->replace_map, repl_obj))
+	if (oidmap_put(the_repository->objects->replace_map, repl_obj))
 		die("duplicate replace ref: %s", refname);
 
 	return 0;
@@ -33,14 +33,16 @@ static int register_replace_ref(const char *refname,
 
 static void prepare_replace_object(void)
 {
-	static int replace_object_prepared;
-
-	if (replace_object_prepared)
+	if (the_repository->objects->replace_map)
 		return;
 
+	the_repository->objects->replace_map =
+		xmalloc(sizeof(*the_repository->objects->replace_map));
+	oidmap_init(the_repository->objects->replace_map, 0);
+
 	for_each_replace_ref(register_replace_ref, NULL);
-	replace_object_prepared = 1;
-	if (!the_repository->objects->replace_map.map.tablesize)
+
+	if (!the_repository->objects->replace_map->map.tablesize)
 		check_replace_refs = 0;
 }
 
@@ -64,7 +66,7 @@ const struct object_id *do_lookup_replace_object(const struct object_id *oid)
 	/* Try to recursively replace the object */
 	while (depth-- > 0) {
 		struct replace_object *repl_obj =
-			oidmap_get(&the_repository->objects->replace_map, cur);
+			oidmap_get(the_repository->objects->replace_map, cur);
 		if (!repl_obj)
 			return cur;
 		cur = &repl_obj->replacement;