[REG 8.0.1 -> 9.0] Git ChangeSelectionDialog doesn't find repo

1. Open a file from a git repository.
   I used README.md from Creator's own repository.
2. Select "Tools" -> "Git" -> "Actions on Commits..." from the menu.
   The ChangeSelectionDialog will open. It reads "Error: Bad working directory." Manually setting a working directory does not work.

Creator 8 automatically detects the working directory from the open file.

Found when updating Squish test tst_git_first_commit.