knitr overwrites tex files it doesn't generate

When you knit x.Rmd into a pdf, x.tex gets overwritten irrespective of whether it's a knitr-generated file. This isn't great on its own, and has caused trouble in the past], but the package author's expectation that you'd read the knitr docs and know that'll happen seems to be a bit much to expect for people clicking the button 'knit' in the RStudio GUI. Please change this behavior.