File changes can trigger the same cell to be evaluated many times

[mbostock]

Because a file can be referenced, either directly or indirectly, by multiple cells, we may end up seeing multiple watch events occur roughly concurrently that cause a single cell to be refreshed many times. We should debounce file watching, related #157, so that these refreshes are coalesced.

[Fil]

I have not been able to reproduce this issue.

Here's my best shot:

```js
const a = [];
```

```js
display(b);
```

```js
const file1 = FileAttachment("double-trouble.txt").text();
```

```js
const file2 = FileAttachment("double-trouble.txt").text();
```

```js
const l1 = display(file1.length);
```

```js
file1;
file2;
l1;
if (a.length > 5) a.splice(0, a.length - 5);
a.push(+new Date() % 100000);
const b = a;
```

I tried with double-trouble.txt as a file or as a data loader, but modifying it never resulted in adding more than 1 timestamp to the b array.

example data "loader":

await new Promise(ok => setTimeout(ok, 1000));
console.log(+Date.now());

[mbostock]

It’s not visible to the client because the runtime coalesces redefinitions within an animation frame. But if you watch the socket events you should see that too many messages are sent when the file changes.

[mbostock]

Fixed in #843.