fix #307 - grunt-watch spawn:false compatibility fix

[vlajos]

if the newly generated part is already in the config file with the same destination update only the source part, instead of add the whole block again. This way the same targets won't be regenerated multiple times if usemin is called multiple times which can happen with grunt-watcher spawn:false mode.

[sindresorhus]

Can you explain the fix and add a test?

[vlajos]

Sorry for the late response.
I try to explain the bug this patch fixes:
If you run grunt-usemin under grunt-watcher with spawn:false, everytime the task is triggered it inherits the config settings from the previous run, so the
outCfg.files = _.union(origCfg.files, cfg.files);
line re-add the same "section" again.
So for example if you use watcher to trigger a build when a file change happens, the first run will be fine, but when the second change happens the generated config files hash will get again the same entries, so it will build everything 2 times, and the third run will build everything 3 times, and so on.
The change I implemented changes the behaviour to not add a new section if the dst file is already there, but overwrite it. And this way it will build everything only 1 times.

I hope this description helped, but let me know If there is any more questions.

I added a test. Probably it is possible to create a shorter but I hope this is more meaningful.

If you run the test for the non-patched version and you add this line before the deepEquals assertion in the new test, you will see what was the original problem:

console.log(config.concat.generated.files);

[vlajos]

I cleaned a bit this pull request. (rebased a few times, and fixed the js linting errors.) I hope the rebases don't cause any trouble.

[vlajos]

ping.

[stephanebachelier]

@vlajos can you add a comment in the source file explaining the reason of the done. The commit message might be sufficient.
If you can add a comment the test explaining that you explicitly call process 2 times.

You need to update your code as it violate the jscs rule "one space before opening curly brace". See build output

[vlajos]

Done.

[stephanebachelier]

LGTM

CC @sindresorhus