test-lib.sh: introduce and use $EMPTY_TREE

This is a special SHA1. Let's keep it at one place, easier to replace
later when the hash change comes, easier to recognize.

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

t/t4010-diff-pathspec.sh

@@ -78,8 +78,6 @@ test_expect_success 'diff-tree pathspec' '
 	test_cmp expected current
 '
 
-EMPTY_TREE=4b825dc642cb6eb9a060e54bf8d69288fbee4904
-
 test_expect_success 'diff-tree with wildcard shows dir also matches' '
 	git diff-tree --name-only $EMPTY_TREE $tree -- "f*" >result &&
 	echo file0 >expected &&