knitr::knit a document with plot code opens existing session plot file

[renkun-ken]

Describe the bug

When r.sessionWatcher is on, when user calls knitr::knit() with a Rmd document with plot commands, the existing plot file stored by R session watcher will be open after the function completes.

To Reproduce

1. Create a Rmd document (e.g. doc1.Rmd) with the following content:

# hello

Hello, world!

```{r}
plot(rnorm(100))
```

2. Call knitr::knit("doc1.Rmd")
3. When the function call completes, no plot file appears.
4. Call plot(rnorm(10)) and the plot file is open. Close this file.
5. Call knitr::knit("doc1.Rmd") again and the previous plot file is open too.