commit: push commit_index update into alloc_commit_node

Whenever we create a commit object via lookup_commit, we
give it a unique index to be used with the commit-slab API.
The theory is that any "struct commit" we create would
follow this code path, so any such struct would get an
index. However, callers could use alloc_commit_node()
directly (and get multiple commits with index 0).

Let's push the indexing into alloc_commit_node so that it's
hard for callers to get it wrong.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

alloc.c

@@ -47,10 +47,18 @@ union any_object {
 
 DEFINE_ALLOCATOR(blob, struct blob)
 DEFINE_ALLOCATOR(tree, struct tree)
-DEFINE_ALLOCATOR(commit, struct commit)
+DEFINE_ALLOCATOR(raw_commit, struct commit)
 DEFINE_ALLOCATOR(tag, struct tag)
 DEFINE_ALLOCATOR(object, union any_object)
 
+void *alloc_commit_node(void)
+{
+	static int commit_count;
+	struct commit *c = alloc_raw_commit_node();
+	c->index = commit_count++;
+	return c;
+}
+
 static void report(const char *name, unsigned int count, size_t size)
 {
 	fprintf(stderr, "%10s: %8u (%"PRIuMAX" kB)\n",
@@ -64,7 +72,7 @@ void alloc_report(void)
 {
 	REPORT(blob, struct blob);
 	REPORT(tree, struct tree);
-	REPORT(commit, struct commit);
+	REPORT(raw_commit, struct commit);
 	REPORT(tag, struct tag);
 	REPORT(object, union any_object);
 }

commit.c

@@ -17,7 +17,6 @@ static struct commit_extra_header *read_commit_extra_header_lines(const char *bu
 int save_commit_buffer = 1;
 
 const char *commit_type = "commit";
-static int commit_count;
 
 static struct commit *check_commit(struct object *obj,
 				   const unsigned char *sha1,
@@ -64,7 +63,6 @@ struct commit *lookup_commit(const unsigned char *sha1)
 	struct object *obj = lookup_object(sha1);
 	if (!obj) {
 		struct commit *c = alloc_commit_node();
-		c->index = commit_count++;
 		return create_object(sha1, OBJ_COMMIT, c);
 	}
 	if (!obj->type)