[RFE] First trigger invocation lists all files, optionally turn this behavior off

I understand watchman's primary purpose is to integrate into a build chain.

However, I've also found it quite well suited to other types of use cases: e.g. notifying users of changes in shared directories, if the `settle` time is sufficiently large (1-5 minutes).

I would understand if the authors don't wish to extend watchman's functionality to cover use cases other than the one originally intended, but the one gap I currently have for this use case is that when watchman is restarted, or a watch is deleted and re-added, the first trigger invocation happens after the settle time has passed, and includes every file in the watch. When the settle time is much longer than one would use for a build chain e.g. measured in minutes this behavior doesn't make sense.

It is simple to ignore the first invocation by checking the value of `WATCHMAN_SINCE` but this will cause any changes before the first invocation to be lost. So I think the simplest way to solve this would be an option to support immediate trigger invocation on creation, ignoring `settle`, so that my handler can ignore that first invocation.

Alternately, an option to directly configure watchman to return only changes since start on the first trigger invocation would work also.