Fix refresh timer in history control to turn off properly

If something had changed while the control was not visible, it would
refresh once and stop the timer.  But if nothing had changed, the timer
would run forever, calling hg via process.Start() every 500ms while
showing the RevisionsInRepositoryView control.
Merge pull request #25 from sillsdev/FixExcessiveHgCalls

Fix refresh timer in history control to turn off properly