t4020: abstract away SHA-1-specific constants

Adjust the test so that it computes variables for blobs instead of using
hard-coded hashes.

Signed-off-by: brian m. carlson <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

t/t4020-diff-external.sh

@@ -13,6 +13,8 @@ test_expect_success setup '
 
 	test_tick &&
 	echo second >file &&
+	before=$(git hash-object file) &&
+	before=$(git rev-parse --short $before) &&
 	git add file &&
 	git commit -m second &&
 
@@ -180,9 +182,13 @@ test_expect_success 'no diff with -diff' '
 echo NULZbetweenZwords | perl -pe 'y/Z/\000/' > file
 
 test_expect_success 'force diff with "diff"' '
+	after=$(git hash-object file) &&
+	after=$(git rev-parse --short $after) &&
 	echo >.gitattributes "file diff" &&
 	git diff >actual &&
-	test_cmp "$TEST_DIRECTORY"/t4020/diff.NUL actual
+	sed -e "s/^index .*/index $before..$after 100644/" \
+		"$TEST_DIRECTORY"/t4020/diff.NUL >expected-diff &&
+	test_cmp expected-diff actual
 '
 
 test_expect_success 'GIT_EXTERNAL_DIFF with more than one changed files' '
@@ -237,7 +243,7 @@ test_expect_success 'diff --cached' '
 	git update-index --assume-unchanged file &&
 	echo second >file &&
 	git diff --cached >actual &&
-	test_cmp "$TEST_DIRECTORY"/t4020/diff.NUL actual
+	test_cmp expected-diff actual
 '
 
 test_expect_success 'clean up crlf leftovers' '