very simple fix for #309

[eddelbuettel]

if no rebuild is needed, still set an on.exit() handler to get back to the
starting directory

[jjallaire]

Let me take a closer look at this before we merge. From first glance it appears that we haven't yet done the setwd in the case of the else clause you put in however clearly it's happening somewhere (perhaps on the native code side?). Want to make sure I fully understand the source of the bug before we apply the fix.

[eddelbuettel]

Sure. I instrumented a local copy with simply i/o to see which setwd() was being triggered (which I'll email you in a minute). That made it pretty apparent that we lacked one in the 'no rebuild' case. But by all means take a closer look; my analysis was clearly of the Sunday-morning variety :)

[jjallaire]

The fix was as expected correct (I had missed that we also set the working directory as when sourcing embedded R code). I was puzzled why we might not have seen this more often in the field but it requires the combination of:

1. Sourcing a file from outside the current directory more then once; and
2. The presence of an embedded R code chunk in the file.

Certainly plausible that this doesn't happen all that often.