ensure changedContent gets cleared

Otherwise this list gets bigger and bigger, not only that there is a
subtle bug. The moment you save a `.css` file we want to create a new
context and start from scratch. However, since the list was never
cleared, it meant that every subsequent save to *any* file (not only
config / css files) creates a new context...

By clearing the least we should work around this problem.

src/cli/build/watching.js

@@ -62,7 +62,7 @@ export function createWatcher(args, { state, rebuild }) {
       extension: path.extname(file).slice(1),
     })
 
-    chain = chain.then(() => rebuild(changedContent))
+    chain = chain.then(() => rebuild(changedContent.splice(0)))
 
     return chain
   }