Rewrite the tests to use a real diff

test/mechanicaldiff.bats

@@ -11,40 +11,21 @@ setup() {
 }
 
 @test "keeps hunk when replacement matches" {
-  run bash -c 'cat <<"EOF" | mechanicaldiff.py foo bar
-diff --git a/file.txt b/file.txt
-index 1111111..2222222 100644
---- a/file.txt
-+++ b/file.txt
-@@ -1,1 +1,1 @@
--foo
-+bar
-EOF'
+  printf "%s\n" "foo" >"$BATS_TEST_TMPDIR/old"
+  printf "%s\n" "bar" >"$BATS_TEST_TMPDIR/new"
+  diff_output="$(git diff --no-index "$BATS_TEST_TMPDIR/old" \
+    "$BATS_TEST_TMPDIR/new" || true)"
+  run mechanicaldiff.py foo bar <<<"$diff_output"
   assert_success
-  expected="$(
-    cat <<'EOF'
-diff --git a/file.txt b/file.txt
-index 1111111..2222222 100644
---- a/file.txt
-+++ b/file.txt
-@@ -1,1 +1,1 @@
--foo
-+bar
-EOF
-  )"
-  assert_output "$expected"
+  assert_output "$diff_output"
 }
 
 @test "drops diff when replacement does not match" {
-  run bash -c 'cat <<"EOF" | mechanicaldiff.py foo bar
-diff --git a/file.txt b/file.txt
-index 1111111..2222222 100644
---- a/file.txt
-+++ b/file.txt
-@@ -1,1 +1,1 @@
--foo
-+baz
-EOF'
+  printf "%s\n" "foo" >"$BATS_TEST_TMPDIR/old"
+  printf "%s\n" "baz" >"$BATS_TEST_TMPDIR/new"
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff" || true
+  run mechanicaldiff.py foo bar <"$BATS_TEST_TMPDIR/diff"
   assert_success
   assert_output ""
 }