t4022: 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 <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4022-diff-rewrite.sh

@@ -13,6 +13,8 @@ test_expect_success setup '
 	  "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM" \
 	  <"$TEST_DIRECTORY"/../COPYING >test &&
 	echo "to be deleted" >test2 &&
+	blob=$(git hash-object test2) &&
+	blob=$(git rev-parse --short $blob) &&
 	git add test2
 
 '
@@ -27,7 +29,7 @@ test_expect_success 'detect rewrite' '
 cat >expect <<EOF
 diff --git a/test2 b/test2
 deleted file mode 100644
-index 4202011..0000000
+index $blob..0000000
 --- a/test2
 +++ /dev/null
 @@ -1 +0,0 @@
@@ -43,7 +45,7 @@ test_expect_success 'show deletion diff without -D' '
 cat >expect <<EOF
 diff --git a/test2 b/test2
 deleted file mode 100644
-index 4202011..0000000
+index $blob..0000000
 EOF
 test_expect_success 'suppress deletion diff with -D' '