fix bug that queries for mergebase too often
fix bug that queries for mergebase too often

Summary:
after we receive an `hg.update` event from watchman, we wait until we receive the next files-changed notification before querying to see if the mergebase changed, to give hg a chance to settle.

we do this by setting `env.finished_an_hg_update`. we were incorrectly only resetting it when the mergebase actually changed, causing us to query for the mergebase after every file change instead of only the one after an `hg.update`!

Changelog: [internal] Fixed a minor bug that queried watchman too often

Reviewed By: jbrown215

Differential Revision: D30622930

fbshipit-source-id: 3105799cf2468ae3b7aee4f151d590337d983276

facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow