GitStatusMonitor tweaks

fixes gitextensions#5439
fixes gitextensions#4256

partly based on gitextensions#5429
The handling was improved in gitextensions#4327, gitextensions#4328, fixing the regression
Those PRs limited the max load for git-status

Changes:
React faster on changing directory (1.0s->0.2s)
Some tweaks to the checks, do not check if update is required if an update is already scheduled

Do not read ignored files due to two problems:
 * Reading ignored can takes significant time. Since this was done the first time, the first update was delayed.
   (Reading ignored could be done later, but this will still delay the next update).
 * Saving all seen ignored files require substantial memory, see gitextensions#4256.
   The list could be limited (see the issue), but that will increase the test time for ignored files

As ignored files will now always trigger git-status may cause some more runs.
However, the ignored file list was never complete, the list was better over time,
so GE must be able to handle this load anyway.