fsmonitor: add new watchman notifications to fsmonitor extension The fsmonitor extension currently sends state-enter and state-leave notifications to watchman on the update operation. This commit creates additional notifications for the following events : - transaction creation and commit/abort. A state-enter notification will be sent when a transaction is created. It will provide the working copy parent's hash. A state-leave notification will be sent when the transaction is committed or aborted. It will provide the working copy parent's hash. - calls to set-parent will cause state-enter and state-leave notifications to be sent. The state-enter notification will be sent prior to the set-parent operation and the working copy parent's hash will be provided at this time. The state-leave notification will be sent after the set-parent operation completes providing the working copy parents hash. Test Plan: tested on dev server to check that necessary notifications were sent/received Differential Revision: https://phab.mercurial-scm.org/D989