Merge branch 'sb/describe-blob'

"git describe $garbage" stopped giving any errors when the garbage
happens to be a string with 40 hexadecimal letters.

* sb/describe-blob:
  describe: confirm that blobs actually exist

builtin/describe.c

@@ -502,7 +502,7 @@ static void describe(const char *arg, int last_one)
 
 	if (cmit)
 		describe_commit(&oid, &sb);
-	else if (lookup_blob(&oid))
+	else if (sha1_object_info(oid.hash, NULL) == OBJ_BLOB)
 		describe_blob(oid, &sb);
 	else
 		die(_("%s is neither a commit nor blob"), arg);

t/t6120-describe.sh

@@ -378,4 +378,12 @@ check_describe tags/A --all A
 check_describe tags/c --all c
 check_describe heads/branch_A --all --match='branch_*' branch_A
 
+test_expect_success 'describe complains about tree object' '
+	test_must_fail git describe HEAD^{tree}
+'
+
+test_expect_success 'describe complains about missing object' '
+	test_must_fail git describe $_z40
+'
+
 test_done