Remote part always considered 'pull step changed' after modifying files in local tree

[林博仁(Buo-ren, Lin)]

I'm using the `ccache` remote part to speed up parts building by making other parts depend on it, however, I noticed that despite the remote part isn't changed at all snapcraft frequently consider its 'pull step changed' and as a result cleaning all the depending part and start building them from pull step again, which is not ideal.

I would expect snapcraft to check the remote part's content before deciding if it is out of date.