project: make getting the .git/index file path more robust

Ahmad Samirrequested to merge
work/ahmad/project-git into master
  • project: don't remove-then-add the same path to QFileSystemWatcher
  • project: add a helper method to get the current GitWidget*
  • project: make getting the .git/index file path more robust
  • project: check if the git widget has been initialized

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