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/builtin-prune.c b/builtin-prune.c
index 6f0ba0d..09864b7 100644
--- a/builtin-prune.c
+++ b/builtin-prune.c
@@ -10,15 +10,10 @@
 
 static int prune_object(char *path, const char *filename, const unsigned char *sha1)
 {
-	char buf[20];
-	const char *type;
-
 	if (show_only) {
-		if (sha1_object_info(sha1, buf, NULL))
-			type = "unknown";
-		else
-			type = buf;
-		printf("%s %s\n", sha1_to_hex(sha1), type);
+		enum object_type type = sha1_object_info(sha1, NULL);
+		printf("%s %s\n", sha1_to_hex(sha1),
+		       (type > 0) ? typename(type) : "unknown");
 		return 0;
 	}
 	unlink(mkpath("%s/%s", path, filename));