t4042: 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/t4042-diff-textconv-caching.sh

@@ -15,9 +15,13 @@ test_expect_success 'setup' '
 	echo bar content 1 >bar.bin &&
 	git add . &&
 	git commit -m one &&
+	foo1=$(git rev-parse --short HEAD:foo.bin) &&
+	bar1=$(git rev-parse --short HEAD:bar.bin) &&
 	echo foo content 2 >foo.bin &&
 	echo bar content 2 >bar.bin &&
 	git commit -a -m two &&
+	foo2=$(git rev-parse --short HEAD:foo.bin) &&
+	bar2=$(git rev-parse --short HEAD:bar.bin) &&
 	echo "*.bin diff=magic" >.gitattributes &&
 	git config diff.magic.textconv ./helper &&
 	git config diff.magic.cachetextconv true
@@ -25,14 +29,14 @@ test_expect_success 'setup' '
 
 cat >expect <<EOF
 diff --git a/bar.bin b/bar.bin
-index fcf9166..28283d5 100644
+index $bar1..$bar2 100644
 --- a/bar.bin
 +++ b/bar.bin
 @@ -1 +1 @@
 -converted: bar content 1
 +converted: bar content 2
 diff --git a/foo.bin b/foo.bin
-index d5b9fe3..1345db2 100644
+index $foo1..$foo2 100644
 --- a/foo.bin
 +++ b/foo.bin
 @@ -1 +1 @@
@@ -59,7 +63,7 @@ test_expect_success 'cached textconv does not run helper' '
 
 cat >expect <<EOF
 diff --git a/bar.bin b/bar.bin
-index fcf9166..28283d5 100644
+index $bar1..$bar2 100644
 --- a/bar.bin
 +++ b/bar.bin
 @@ -1,2 +1,2 @@
@@ -67,7 +71,7 @@ index fcf9166..28283d5 100644
 -converted: bar content 1
 +converted: bar content 2
 diff --git a/foo.bin b/foo.bin
-index d5b9fe3..1345db2 100644
+index $foo1..$foo2 100644
 --- a/foo.bin
 +++ b/foo.bin
 @@ -1,2 +1,2 @@
@@ -84,7 +88,7 @@ test_expect_success 'changing textconv invalidates cache' '
 
 cat >expect <<EOF
 diff --git a/bar.bin b/bar.bin
-index fcf9166..28283d5 100644
+index $bar1..$bar2 100644
 --- a/bar.bin
 +++ b/bar.bin
 @@ -1,2 +1,2 @@
@@ -92,7 +96,7 @@ index fcf9166..28283d5 100644
 -converted: bar content 1
 +converted: bar content 2
 diff --git a/foo.bin b/foo.bin
-index d5b9fe3..1345db2 100644
+index $foo1..$foo2 100644
 --- a/foo.bin
 +++ b/foo.bin
 @@ -1 +1 @@