t3905: 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/t3905-stash-include-untracked.sh

@@ -35,24 +35,26 @@ test_expect_success 'stash save --include-untracked cleaned the untracked files'
 	test_cmp expect actual
 '
 
+tracked=$(git rev-parse --short $(echo 1 | git hash-object --stdin))
+untracked=$(git rev-parse --short $(echo untracked | git hash-object --stdin))
 cat > expect.diff <<EOF
 diff --git a/HEAD b/HEAD
 new file mode 100644
-index 0000000..d00491f
+index 0000000..$tracked
 --- /dev/null
 +++ b/HEAD
 @@ -0,0 +1 @@
 +1
 diff --git a/file2 b/file2
 new file mode 100644
-index 0000000..d00491f
+index 0000000..$tracked
 --- /dev/null
 +++ b/file2
 @@ -0,0 +1 @@
 +1
 diff --git a/untracked/untracked b/untracked/untracked
 new file mode 100644
-index 0000000..5a72eb2
+index 0000000..$untracked
 --- /dev/null
 +++ b/untracked/untracked
 @@ -0,0 +1 @@
@@ -109,10 +111,11 @@ test_expect_success 'stash save -u dirty index' '
 	git stash -u
 '
 
+blob=$(git rev-parse --short $(echo 4 | git hash-object --stdin))
 cat > expect <<EOF
 diff --git a/file3 b/file3
 new file mode 100644
-index 0000000..b8626c4
+index 0000000..$blob
 --- /dev/null
 +++ b/file3
 @@ -0,0 +1 @@