t4030: 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/t4030-diff-textconv.sh

@@ -148,7 +148,8 @@ test_expect_success 'diffstat does not run textconv' '
 # restore working setup
 echo file diff=foo >.gitattributes
 
-cat >expect.typechange <<'EOF'
+symlink=$(git rev-parse --short $(printf frotz | git hash-object --stdin))
+cat >expect.typechange <<EOF
 --- a/file
 +++ /dev/null
 @@ -1,2 +0,0 @@
@@ -156,7 +157,7 @@ cat >expect.typechange <<'EOF'
 -1
 diff --git a/file b/file
 new file mode 120000
-index 0000000..67be421
+index 0000000..$symlink
 --- /dev/null
 +++ b/file
 @@ -0,0 +1 @@