Fix changes sometimes disappearing entirely

```git-check-assertions
run test/mechanicaldiff.bats
assert_success

git checkout HEAD~ bin
run test/mechanicaldiff.bats
assert_failure
assert_output --partial "not ok 9 keeps matching change with an extra removed line"
assert_output --partial "not ok 10 keeps matching change with an extra added line"
```

bin/mechanicaldiff

@@ -40,11 +40,13 @@ def filter_change_block(
     if trim_blank_ends(transformed_removed) == trim_blank_ends(added_lines):
         return change_lines
 
-    if len(removed_lines) != len(added_lines):
-        return []
-
     kept_lines = []
-    for removed, added in zip(removed_lines, added_lines):
+    max_len = max(len(removed_lines), len(added_lines))
+    for idx in range(max_len):
+        removed = removed_lines[idx] if idx < len(removed_lines) else None
+        added = added_lines[idx] if idx < len(added_lines) else None
+        if removed is None:
+            break
         if re.sub(search, replace, removed) == added:
             kept_lines.append(f"-{removed}")
             kept_lines.append(f"+{added}")

default.nix

@@ -7,6 +7,7 @@
   python3Packages,
   shellcheck-minimal,
   shfmt,
+  patchutils_0_4_2,
 }:
 
 let
@@ -42,6 +43,7 @@ stdenv.mkDerivation {
     python3Packages.flake8
     shellcheck-minimal
     shfmt
+    patchutils_0_4_2
   ];
 
   checkPhase = ''

flake.nix

@@ -26,6 +26,7 @@
             pkgs.python3Packages.flake8
             pkgs.shellcheck
             pkgs.shfmt
+            pkgs.patchutils_0_4_2
           ];
         };
       }

test/mechanicaldiff.bats

@@ -162,6 +162,71 @@ setup() {
   assert_output "$(cat "$BATS_TEST_TMPDIR/diff")"
 }
 
+@test "keeps matching change with an extra removed line" {
+  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
+	foo
+	bar
+	baz
+	EOF
+  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
+	foo_changed
+	baz
+	EOF
+
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff" || true
+
+  cat >"$BATS_TEST_TMPDIR/new_kept" <<-'EOF'
+	foo_changed
+	bar
+	baz
+	EOF
+  git diff --no-index "$BATS_TEST_TMPDIR/old" \
+    "$BATS_TEST_TMPDIR/new_kept" \
+    >"$BATS_TEST_TMPDIR/diff_expected" || true
+
+  run mechanicaldiff foo foo_changed <"$BATS_TEST_TMPDIR/diff"
+  assert_failure
+  expected="$(sed \
+    -e 's/new_kept/new/g' \
+    -e '/^index /d' \
+    "$BATS_TEST_TMPDIR/diff_expected")"
+  output_normalized="$(printf '%s\n' "$output" | recountdiff | sed -e '/^index /d')"
+  assert_equal "$expected" "$output_normalized"
+}
+
+@test "keeps matching change with an extra added line" {
+  cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'
+	foo
+	baz
+	EOF
+  cat >"$BATS_TEST_TMPDIR/new" <<-'EOF'
+	foo_changed
+	bar
+	baz
+	EOF
+
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff" || true
+
+  cat >"$BATS_TEST_TMPDIR/new_kept" <<-'EOF'
+	foo_changed
+	baz
+	EOF
+  git diff --no-index "$BATS_TEST_TMPDIR/old" \
+    "$BATS_TEST_TMPDIR/new_kept" \
+    >"$BATS_TEST_TMPDIR/diff_expected" || true
+
+  run mechanicaldiff foo foo_changed <"$BATS_TEST_TMPDIR/diff"
+  assert_failure
+  expected="$(sed \
+    -e 's/new_kept/new/g' \
+    -e '/^index /d' \
+    "$BATS_TEST_TMPDIR/diff_expected")"
+  output_normalized="$(printf '%s\n' "$output" | recountdiff | sed -e '/^index /d')"
+  assert_equal "$expected" "$output_normalized"
+}
+
 @test "ignores leading and trailing blank lines for comparison" {
   cat >"$BATS_TEST_TMPDIR/old" <<-'EOF'