Use blob_, commit_, tag_, and tree_type throughout.

This replaces occurences of "blob", "commit", "tag", and "tree",
where they're really used as type specifiers, which we already
have defined global constants for.

Signed-off-by: Peter Eriksen <s022018@student.dtu.dk>
Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/combine-diff.c b/combine-diff.c
index f17aab3..7693884 100644
--- a/combine-diff.c
+++ b/combine-diff.c
@@ -1,5 +1,6 @@
 #include "cache.h"
 #include "commit.h"
+#include "blob.h"
 #include "diff.h"
 #include "diffcore.h"
 #include "quote.h"
@@ -104,7 +105,7 @@
 		return xcalloc(1, 1);
 	}
 	blob = read_sha1_file(sha1, type, size);
-	if (strcmp(type, "blob"))
+	if (strcmp(type, blob_type))
 		die("object '%s' is not a blob!", sha1_to_hex(sha1));
 	return blob;
 }