Fail to create pull request

[skabashnyuk]

Description

Not able to create pull request.

Reproduction Steps

1. Clone your github repository. Example: https://github.com/skabashnyuk/petclinic
2. Checkout to some branch. Example fix
3. Commit some changes.
4. Try to create PR.
   

[skabashnyuk]

Couse of the issue is a fact that during panel initialization fired 4 PartStackStateChangedEvent simultaneously. Those 4 events generate 4 org.eclipse.che.plugin.pullrequest.client.workflow.Context with the wrong state inside because of this code from WorkflowExecutor class

  public void init(final VcsHostingService vcsHostingService, final ProjectConfig project) {
    final Optional<Context> contextOpt = getContext(project.getName());
    if (!contextOpt.isPresent()) {
      doInit(vcsHostingService, project);
    } else {
      checkVcsState(contextOpt.get())
          .then(
              new Operation<Boolean>() {
                @Override
                public void apply(Boolean stateChanged) throws OperationException {
                  if (stateChanged) {
                    invalidateContext(contextOpt.get().getProject());
                    doInit(vcsHostingService, project);
                  } else {
                    eventBus.fireEvent(new CurrentContextChangedEvent(contextOpt.get()));
                  }
                }
              });
    }
  }

are not adapted for a 4 sequentially events.

[RomanNikitenko]

I reproduced the bug.
In my case I watch that SelectionChangedEvent is fired 4 times simultaneously.