This bugfix implements correct way to calculate source sha1 hash, before it will changed,
by propagation of unchanged sources.
This commit will also include regenerated static files and *.hash files.
Clearly we were just able to read the content via ioutil.ReadFile but for consistency I think it would be better to call check(err) after the call to os.Open("/tmp/dat") on line 34.
Tools like errcheck (https://github.com/kisielk/errcheck) will complain about uncaught / unhandled err's. Also can we really be sure some 3rd party process didn't nuke '/tmp' during execution :-)
