Fix multiple grammars not always recompile issue #789

[huacnlee]

Closes #789

[tomtau]

LGTM, so the root cause was that only a single path was passed to that function at a time which prevented regenerating the other paths (only one of them was tracked)?

[huacnlee]

Yes, I have tested these changes with cargo watch -x test.

I also test that behavior (Before and After changes) with cargo test or cargo build, which does not appear.

So, it's looks like this bug only appears in cargo watch.

[huacnlee]

I realize that maybe we don't need set all of pest file contents as a value, just only give a file updated at or a content md5. When the .pest files has changed, it's also can trigger change events.

Ignore that, it not a serious problem. And fs::metadata modified might not be available on all platforms.

[NoahTheDuke]

Looks like a solid fix to me.

[jayhale]

Thank you!