Fix: Ensure not to start more than one evaluation at a time.
Fix: Ensure not to start more than one evaluation at a time.

Even though preceding optimizations target to minimize latencies of evaluating
file modifications it might happen especially with large files being read from
a slow maybe network file system, that the next run is already triggered,
before the current evaluation has finished.

This causes two or more promise chains to be active at a time with unknown
consequences.

As the structure of the promise chain makes aborting the process hard to
implement, this commit simply skips events as long as the chain is busy.

jisaacks/GitGutter