allow watchDepFile to be called before watchFile

If `watchDepFile(mfile, file)` is called before `watchFile(mfile)` is called, then then `file` isn't watched for changes.

This can cause problems with transforms, for example:

clone https://gist.github.com/ansis/575164c95d8ba1580c0f
run `watchify -t brfs index.js -o out`
edit `beep` // out is updated
edit `beep` again // out is not updated this time


Is this the right way to fix this?