Add support for rewriting hunks if only specific lines match

test/mechanicaldiff.bats

@@ -128,3 +128,31 @@ setup() {
   assert_success
   assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
 }
+
+@test "keeps only matching line in a consecutive change block" {
+  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
+	foo
+	bar
+	baz
+	EOF
+  sed \
+    -e 's/foo/foo_changed/' \
+    -e 's/bar/bar_changed/' \
+    "$BATS_TEST_TMPDIR/old" >"$BATS_TEST_TMPDIR/new"
+  sed 's/foo/foo_changed/' "$BATS_TEST_TMPDIR/old" \
+    >"$BATS_TEST_TMPDIR/new_kept"
+
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff_full" || true
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new_kept" \
+    >"$BATS_TEST_TMPDIR/diff_expected" || true
+
+  run mechanicaldiff.py foo foo_changed <"$BATS_TEST_TMPDIR/diff_full"
+  assert_failure
+  expected="$(sed \
+    -e 's/new_kept/new/g' \
+    -e '/^index /d' \
+    "$BATS_TEST_TMPDIR/diff_expected")"
+  output_normalized="$(printf '%s\n' "$output" | sed -e '/^index /d')"
+  assert_equal "$expected" "$output_normalized"
+}