Introduce `AddSafeDirectory` action which, when dispatched, will call
the `GitPanel.add_safe_directory` method. This method will take the
absolute path of the current worktree, as it currently is not written to
support multiple, or none, worktreess, and will call `git config
--global --add safe.directory [DIRECTORY]` with that path, to ensure
that, even if the user does not own the repository's `.git/` folder,
they'll still be able to read its contents.
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