convert object type handling from a string to a number

We currently have two parallel notation for dealing with object types
in the code: a string and a numerical value.  One of them is obviously
redundent, and the most used one requires more stack space and a bunch
of strcmp() all over the place.

This is an initial step for the removal of the version using a char array
found in object reading code paths.  The patch is unfortunately large but
there is no sane way to split it in smaller parts without breaking the
system.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/tag.c b/tag.c
index 864ac1b..30ffc17 100644
--- a/tag.c
+++ b/tag.c
@@ -85,18 +85,18 @@
 
 int parse_tag(struct tag *item)
 {
-	char type[20];
+	enum object_type type;
 	void *data;
 	unsigned long size;
 	int ret;
 
 	if (item->object.parsed)
 		return 0;
-	data = read_sha1_file(item->object.sha1, type, &size);
+	data = read_sha1_file(item->object.sha1, &type, &size);
 	if (!data)
 		return error("Could not read %s",
 			     sha1_to_hex(item->object.sha1));
-	if (strcmp(type, tag_type)) {
+	if (type != OBJ_TAG) {
 		free(data);
 		return error("Object %s not a tag",
 			     sha1_to_hex(item->object.sha1));