Add option to ignore paths when setting watches.

[imsodin]

This is a modified version of Zillode#3 and adds (some of) the capability requested in #79. syncthing-inotify uses the mentioned fork with this functionality and it works fine. The main advantage over the status quo (filtering events out after generation) is that this saves watch handles (and is slightly more efficient).

The changes are completely backward compatible. Nothing changes for the existing Watch function. The new functionality is included in a new function WatchWithIgnoring. This function takes the function variable doNotWatch in addition to the usual arguments. This function should return true if given a path (string) that is to be ignored. This is implemented by returning different functions in recFunc depending on whether Watch (no change) or WatchWithIgnoring (check doNotWatch at beginning) is used.

I would appreciate your feedback.

[ppknap]

We cannot merge this PR since it is not cross-platform - doNotWatch functor will not work on pure-recursive watchers like Windows's ReadDirectoryChangesW.

[imsodin]

Thanks for responding!
I understand that my implementation is only partial (i.e. only having an effect on nonrecursive trees and behaving just like a normal watch on recursive ones) and thus not mergeable. I looked at the recursive tree and it looks like it would be much more involved to do something similar there. I don't really have time right now to look into that.

[ppknap]

@imsodin I'm closing this PR for now. We can reopen it again when a cross-platform solution is implemented.