Make hash-object more robust against malformed objects

Commits, trees and tags have structure. Don't let users feed git
with malformed ones. Sooner or later git will die() when
encountering them.

Note that this patch does not check semantics. A tree that points
to non-existent objects is perfectly OK (and should be so, users
may choose to add commit first, then its associated tree for example).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1007-hash-object.sh

@@ -188,4 +188,17 @@ for args in "-w --stdin-paths" "--stdin-paths -w"; do
 	pop_repo
 done
 
+test_expect_success 'corrupt tree' '
+	echo abc >malformed-tree
+	test_must_fail git hash-object -t tree malformed-tree
+'
+
+test_expect_success 'corrupt commit' '
+	test_must_fail git hash-object -t commit --stdin </dev/null
+'
+
+test_expect_success 'corrupt tag' '
+	test_must_fail git hash-object -t tag --stdin </dev/null
+'
+
 test_done