Add a test where all pairs in a hunk become context

test/mechanicaldiff.bats

@@ -156,3 +156,22 @@ setup() {
   output_normalized="$(printf '%s\n' "$output" | sed -e '/^index /d')"
   assert_equal "$expected" "$output_normalized"
 }
+
+@test "drops hunk when all pairs become context" {
+  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" >"$BATS_TEST_TMPDIR/new"
+
+  git diff --no-index "$BATS_TEST_TMPDIR/old" "$BATS_TEST_TMPDIR/new" \
+    >"$BATS_TEST_TMPDIR/diff_full" || true
+
+  run mechanicaldiff.py qux quux <"$BATS_TEST_TMPDIR/diff_full"
+  assert_failure
+  assert_output ""
+}