This commit aggregates the following changes:
1. Fix the format (i.e., indentation) when printing stop hooks via `target
stop-hook list`.
2. Add `IndentScope Stream::MakeIndentScope()` to make managing (and restoring!)
of the indentation level on `Stream` instances more ergonomic and less error
prone.
3. Simplify printing of stop hooks using the new `IndentScope`.
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