TortoiseGitProc and TGitCache crash on repositories with broken core.worktree path
**What steps will reproduce the problem?** 1. Have a repository with (a submodule and) `.git` file pointing to the superproject 2. Rename the folder in explorer 3. Refresh the view or open TortoiseGitProc **What is the expected output? What do you see instead?** TGitCache crashes, but restarts immediately. **What version of TortoiseGit and Git are you using? On what operating system?** TortoiseGit 2.9.0.0
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